rt2x00: Make rt2x00_set/get_field macros

The rt2x00_set_field functions are very often used,
but GCC is better able to optimize them when they
are macros instead of static inline functions.

After changing it to macro's each rt2x00 driver will
loose about ~3500 bytes in size.

Signed-off-by: Ivo van Doorn <IvDoorn@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>