wifi: mac80211_hwsim: fix TX link selection

Now that we have a pointer to the TX STA even when it's
not authenticated/... yet, fix the TX link selection in
hwsim to select only among the valid links for the STA,
requiring a STA pointer here. Also implement a simple
round-robin between links to make life more interesting.

While at it, also consider A3 when translating to link
addresses.

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