wl12xx: disable 11a channels when wl->enable_11a is known

Disabling the 11a channels when not supported in the reg_notify function was
not working as it should, because when the driver is initiailizing (and
registering itself with mac80211), it would get the reg notification too
early.  At that point the driver wouldn't have received the NVS yet, so it
wouldn't know whether 11a was supported.

To fix this, we disable 11a channels when we read the NVS instead.  Also, it
is easier (and still safe) to set n_channels to zero instead of setting the
disabled flag on every 11a channel.

Signed-off-by: Luciano Coelho <luciano.coelho@nokia.com>