mac80211: avoid useless memory write on each frame RX

In the likely case that probe_count is 0, don't write to the
memory there.

Also use ifmgd consistently in the function, instead of using
sdata->u.mgd as well.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>