wl1271: Enable 11a support always, prevent scanning for unsupporting chips

This patch always enables 11a band towards the mac80211, but prevents scanning
(and hence the usage of) 11a band channels if the chipset does not support it.

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