MAINTAINERS: Adjust file entry in LOONGSON SECURITY ENGINE DRIVERS

Commit 5c83b07d ("tpm: Add a driver for Loongson TPM device") adds a
driver at drivers/char/tpm/tpm_loongson.c, and commit 74fddd5f
("MAINTAINERS: Add entry for Loongson Security Engine drivers") adds a new
section LOONGSON SECURITY ENGINE DRIVERS intending to refer to that driver.
It however adds the entry drivers/char/tpm_loongson.c; note that it misses
the tpm subdirectory.

Adjust the entry to refer to the intended file.

Fixes: 74fddd5f ("MAINTAINERS: Add entry for Loongson Security Engine drivers")
Signed-off-by: Lukas Bulwahn <lukas.bulwahn@redhat.com>
Signed-off-by: Lee Jones <lee@kernel.org>