wifi: rt2x00: use explicitly signed or unsigned types

On some platforms, `char` is unsigned, but this driver, for the most part,
assumed it was signed.  In other places, it uses `char` to mean an
unsigned number, but only in cases when the values are small.  And in
still other places, `char` is used as a boolean.  Put an end to this
confusion by declaring explicit types, depending on the context.

Link: https://lkml.kernel.org/r/20221019155541.3410813-1-Jason@zx2c4.com


Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>