cfg80211/mac80211: report signal strength for mgmt frames

Add the signal strength (in dBm only for now) to
frames that are received via nl80211's various
frame APIs.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Acked-by: Kalle Valo <kvalo@qca.qualcomm.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>