rt2x00: Move rt73usb register access wrappers into rt2x00usb

rt2500usb and rt73usb have different register word sizes,
for that reason the register access wrappers were never
moved into rt2x00usb.
With rt2800usb on its way, we should favor the 32bit
register access and move those wrappers into rt2x00usb.
That saves duplicate code, since only rt2500usb will
need the special 16bit wrappers.

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