mac80211: add last beacon time in scan list

This patch adds the interval between the scan results and the last time a
beacon was received in the result of the scan.

Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>
Signed-off-by: Tomas Winkler <tomas.winkler@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>