rust: firmware: Use `ffi::c_char` type in `FwFunc`

The `FwFunc` struct contains an function with a char pointer argument,
for which a `*const u8` pointer was used. This is not really the
"proper" type for this, so use a `*const kernel::ffi::c_char` pointer
instead.

This has no real functionality changes, since now `kernel::ffi::c_char`
(which bindgen uses for `char`) is now a type alias to `u8` anyways,
but before commit 1bae8729 ("rust: map `long` to `isize` and `char`
to `u8`") the concrete type of `kernel::ffi::c_char` depended on the
architecture (However all supported architectures at the time mapped to
`i8`).

This caused problems on the v6.13 tag when building for 32 bit arm (with
my patches), since back then `*const i8` was used in the function
argument and the function that bindgen generated used
`*const core::ffi::c_char` which Rust mapped to `*const u8` on 32 bit
arm. The stable v6.13.y branch does not have this issue since commit
1bae8729 ("rust: map `long` to `isize` and `char` to `u8`") was
backported.

This caused the following build error:
```
error[E0308]: mismatched types
  --> rust/kernel/firmware.rs:20:4
   |
20 |         Self(bindings::request_firmware)
   |         ---- ^^^^^^^^^^^^^^^^^^^^^^^^^^ expected fn pointer, found fn item
   |         |
   |         arguments to this function are incorrect
   |
   = note: expected fn pointer `unsafe extern "C" fn(_, *const i8, _) -> _`
                 found fn item `unsafe extern "C" fn(_, *const u8, _) -> _ {request_firmware}`
note: tuple struct defined here
  --> rust/kernel/firmware.rs:14:8
   |
14 | struct FwFunc(
   |        ^^^^^^

error[E0308]: mismatched types
  --> rust/kernel/firmware.rs:24:14
   |
24 |         Self(bindings::firmware_request_nowarn)
   |         ---- ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ expected fn pointer, found fn item
   |         |
   |         arguments to this function are incorrect
   |
   = note: expected fn pointer `unsafe extern "C" fn(_, *const i8, _) -> _`
                 found fn item `unsafe extern "C" fn(_, *const u8, _) -> _ {firmware_request_nowarn}`
note: tuple struct defined here
  --> rust/kernel/firmware.rs:14:8
   |
14 | struct FwFunc(
   |        ^^^^^^

error[E0308]: mismatched types
  --> rust/kernel/firmware.rs:64:45
   |
64 |         let ret = unsafe { func.0(pfw as _, name.as_char_ptr(), dev.as_raw()) };
   |                            ------           ^^^^^^^^^^^^^^^^^^ expected `*const i8`, found `*const u8`
   |                            |
   |                            arguments to this function are incorrect
   |
   = note: expected raw pointer `*const i8`
              found raw pointer `*const u8`

error: aborting due to 3 previous errors
```

Fixes: de658283 ("rust: add firmware abstractions")
Cc: stable@vger.kernel.org
Reviewed-by: Benno Lossin <benno.lossin@proton.me>
Signed-off-by: Christian Schrefl <chrisi.schrefl@gmail.com>
Acked-by: Miguel Ojeda <ojeda@kernel.org>
Link: https://lore.kernel.org/r/20250413-rust_arm_fix_fw_abstaction-v3-1-8dd7c0bbcd47@gmail.com


[ Add firmware prefix to commit subject. - Danilo ]
Signed-off-by: Danilo Krummrich <dakr@kernel.org>