mac80211: mesh: drop frames appearing to be from us

If there are multiple mesh stations with the same MAC address,
they will both get confused and start throwing warnings.

Obviously in this case nothing can actually work anyway, so just
drop frames that look like they're from ourselves early on.

Reported-by: Gui Iribarren <gui@altermundi.net>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>