rt2x00: Disable RX when switching antenna

Antenna switching will be ignored when RX is enabled
during the switch. Make sure we disable the RX during
the switch and don't forget to reenable it later.

Signed-off-by: Ivo van Doorn <IvDoorn@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>
Signed-off-by: David S. Miller <davem@davemloft.net>