mac80211: move 4-addr sta pointer clearing before synchronize_rcu()

The pointer should be cleared before synchronize_rcu() so that the
consequently dead station won't be found by any lookups in the TX
or RX paths.

Also check that the station is actually the one being removed, the
check is not needed because each 4-addr VLAN can only have a single
station and non-4-addr VLANs always have a NULL pointer there, but
the code is clearer this way (and we avoid the memory write.)

Signed-off-by: Johannes Berg <johannes.berg@intel.com>