mac80211: add an option for drivers to check if packets can be aggregated

Some hardwares have limitations on the packets' type in AMSDU.
Add an optional driver callback to determine if two skbs can
be used in the same AMSDU or not.

Signed-off-by: Sara Sharon <sara.sharon@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>