mac80211: add beacon timestamp to beacon template in IBSS

This patch adds a beacon timestamp to the beacon template used in IBSS
mode. This way the underlying driver can update its TSF accordingly.
According the spec station should adopt the highest TSF from an incoming
beacons in the cell.

Signed-off-by: Assaf Krauss <assaf.krauss@intel.com>
Signed-off-by: Zhu Yi <yi.zhu@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>