wl1271: use per-channel max tx power passed by mac80211 when scanning

We were always using the max transmit power when scanning.  Now we use the
values passed to the driver by the mac80211 stack, so that we comply with
regulations.

Signed-off-by: Luciano Coelho <luciano.coelho@nokia.com>
Reviewed-by: Saravanan Dhanabal <ext-saravanan.dhanabal@nokia.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>