wifi: mac80211: fix qos on mesh interfaces

When ieee80211_select_queue is called for mesh, the sta pointer is usually
NULL, since the nexthop is looked up much later in the tx path.
Explicitly check for unicast address in that case in order to make qos work
again.

Cc: stable@vger.kernel.org
Fixes: 50e2ab39

 ("wifi: mac80211: fix queue selection for mesh/OCB interfaces")
Signed-off-by: Felix Fietkau <nbd@nbd.name>
Link: https://lore.kernel.org/r/20230314095956.62085-1-nbd@nbd.name
Signed-off-by: Johannes Berg <johannes.berg@intel.com>