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

drivers/net/wireless/rt2x00/Kconfig

 menuconfig RT2X00
-	bool "Ralink driver support"
+	tristate "Ralink driver support"
 	depends on MAC80211 && WLAN_80211 && EXPERIMENTAL
 	---help---
 	  This will enable the experimental support for the Ralink drivers,