nios2: uapi: Remove custom asm/swab.h from UAPI

UAPI headers are not supposed to leak references to kconfig symbols.
They are undefined there in any case. As all actual definitions of this
header are guarded behind a kconfig symbol, for userspace the header is
always identical to its asm-generic variant.

Make the custom UAPI header a kernel-internal one, so the leaks of
kconfig symbols are fixed and userspace will instead use
asm-generic/swab.h directly.

Suggested-by: Arnd Bergmann <arnd@arndb.de>
Link: https://lore.kernel.org/lkml/93c55086-931a-4282-a94c-de4954047fa9@app.fastmail.com/


Signed-off-by: Thomas Weißschuh <thomas.weissschuh@linutronix.de>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>