wl1251: initialise default channel to zero

Because wl->channel was initialised to one, the first join command in
wl1251_op_config() always failed.

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>