mac80211: allow drivers to request DTIM period

Some features require knowing the DTIM period
before associating. This implements the ability
to wait for a beacon in mac80211 before assoc
to provide this value. It is optional since
most likely not all drivers will need this.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>