wifi: mac80211: clarify vif handling in TX dequeue

The vif pointer at least looks like it can actually be NULL
in some cases such as the monitor-mode vif, causing static
checkers to complain with the immediate derefence. In these
cases the sta pointer will also be NULL, but clarify it in
the code anyway.

Link: https://msgid.link/20240131164910.60066625a239.Idfb6a5a9876f9f631eae760055e1c4018259a971@changeid


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