WNM: Follow BTM procedure if the last link is dropped

WNM: Follow BTM procedure if the last link is dropped

If the last link is dropped, it makes sense to follow the BTM procedure.
However, in that case we need to prevent reconnection to this link
specifically, while if the AP MLD is terminating we need to forbid
connecting to the AP MLD.

As such, add a new variable to track the BSSID or AP MLD MAC address.
Which one it refers to depends on whether wnm_link_removal is set.

This also simplifies the check in wnm_is_bss_excluded() and untangles it
from wpa_s->current_bss.

Signed-off-by: Benjamin Berg <benjamin.berg@intel.com>