mac80211: redesign scan RX

Scan receive is rather inefficient when there are
multiple virtual interfaces. We iterate all of the
virtual interfaces and then notify cfg80211 about
each beacon many times.

Redesign scan RX to happen before everything else.
Then we can also get rid of IEEE80211_RX_IN_SCAN
since we don't have to accept frames into the RX
handlers for scanning or scheduled scanning any
more. Overall, this simplifies the code.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>