wl1251: add channel to wl1251_cmd_join() parameters

Because join channel tunes to a channel, better to make it more obvious
by adding a parameter for it.

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>