rt2x00: Block adhoc & master mode

rt2x00 is broken when it comes down to adhoc and master mode.
The main problem is the beaconing, which is completely failing.
Untill a solution has been found, both beacon requiring modes
must be disabled to prevent numerous bug reports.

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