wlcore: don't switch channels on disconnected STA vifs

Sending the FW a channel switch command on a disconnected
vif may result in a beacon loss event. Avoid this corner case.

Signed-off-by: Arik Nemtsov <arik@wizery.com>
Signed-off-by: Eliad Peller <eliad@wizery.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>