mac80211: fix dot11MulticastTransmittedFrameCount tested address

dot11MulticastTransmittedFrameCount should be updated according
to the DA, which might be different from A1. Checking A1 results
in the counter being 0 in case of station, as to-DS data frames
use A1 for the BSSID.

This behaviour is defined in state machines, specifically in the
sta_tx_dcf_3.1d(10) description of 802.11-2012.

Signed-off-by: Eliad Peller <eliad@wizery.com>
[rewrite commit message]
Signed-off-by: Johannes Berg <johannes.berg@intel.com>