wl1251: fix channel switching in monitor mode

Use the ENABLE_RX command for channel switching when no interface is present
(monitor mode only).
The advantage of ENABLE_RX is that it leaves the tx data path disabled in
firmware, whereas the usual JOIN command seems to transmit some frames at
firmware level.

Signed-off-by: David Gnedt <david.gnedt@davizone.at>
Signed-off-by: Pali Rohár <pali.rohar@gmail.com>
Signed-off-by: Pavel Machek <pavel@ucw.cz>
Signed-off-by: John W. Linville <linville@tuxdriver.com>