wl1251: use mac80211 provided channel parameters in scanning

The number of channels to be used in scan was hard coded in wl1251. The
proper way is to use the channels array provided by mac80211.

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