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.

Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Cc: Stanislaw Gruszka <stf_xl@wp.pl>
Cc: Helmut Schaa <helmut.schaa@googlemail.com>
Cc: Kalle Valo <kvalo@kernel.org>
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
Acked-by: Stanislaw Gruszka <stf_xl@wp.pl>
Signed-off-by: Kalle Valo <kvalo@kernel.org>
Link: https://lore.kernel.org/r/20221019155541.3410813-1-Jason@zx2c4.com