rt2x00: Fix build error when mac80211=M rt2x00=Y

Make menuconfig RT2X00 a tristate instead of boolean,
otherwise we do not correctly inherit the mac80211 value
on which RT2X00 depends, and makes it possible to
compile rt2x00 into the kernel while mac80211 is a
module.

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