mac80211: allow low level drivers to report packet loss

Add API that allows low level drivers to notify mac80211 about TX
packet loss. This is useful when there are FW triggers to notify the
low level driver about these events.

Signed-off-by: Arik Nemtsov <arik@wizery.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>