at76c50x: fix scan does not work with latest mac80211

since commit 3afc2167 scan in not
working anymore, due to mac80211 requires rx frequency status
information.

This patch makes the driver report this information.

While NOT scanning this is straightforward.
While scanning the firmware performs RF sweep and we cannot track
the actual tuning frequency, so this is guessed by parsing beacons
and probe responses.
This should be enough for ensuring functionality.

Thanks-to: Johannes Berg <johannes@sipsolutions.net> [ for suggestions and reviewing ]
Signed-off-by: Andrea Merello <andrea.merello@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>