rt2x00: Cleanup Makefile

Simplify the way rt2x00 assigns new objects to the
rt2x00lib module. This saves a few if statements
and overall does this looks much nicer.

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