wl12xx: disable 11a channels when regulatory changes if 11a is not supported

Instead of simply not scanning for the 11a channels when not supported by the
hardware, disable the channels in reg_notify.  This centralizes the decision
on whether to scan 5GHz channel in one place and allows userspace to know
exactly which channels are in use.

Based on Juuso Oikarinen's idea.

Cc: Juuso Oikarinen <juuso.oikarinen@nokia.com>
Signed-off-by: Luciano Coelho <luciano.coelho@nokia.com>
Reviewed-by: Juuso Oikarinen <juuso.oikarinen@nokia.com>