wifi: mac80211_hwsim: implement sta_state for MLO

In MLO, we need to transmit to another MLD and select
the link to it, which requires knowing the station.
But in TX, mac80211 will not give us a station that's
not added to the driver, which in the older add/remove
API is only done later.

Implement the new API in MLO so we know about the STA
at all times and get a pointer during TX as well.

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