wl1251: create wl1251_join()

Better to use wl1251_cmd_join() only for sending the command and move
the logic to wl1251_join().

Signed-off-by: Kalle Valo <kalle.valo@nokia.com>
Reviewed-by: Vidhya Govindan <vidhya.govindan@nokia.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>