wl1251: fix channel setting in wl1251_op_config()

There is a bug in wl1251_op_config(). It was calling join with previous
channel. Fix it by setting assigning wl->channel before calling join command.

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>