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>