cfg80211: Clear "beacon_found" on regulatory restore

Restore the default state to the "beacon_found" flag when
the channel flags are restored.  Otherwise, we can end up
with a channel that we can no longer transmit on even when
we can see beacons on that channel.

Signed-off-by: Paul Stewart <pstew@chromium.org>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>