mac80211: MBSSID support in interface handling

Configure multiple BSSID and enhanced multi-BSSID advertisement (EMA)
parameters in mac80211 for AP mode.

For each interface, 'mbssid_tx_vif' points to the transmitting interface of
the MBSSID set. The pointer is set to NULL if MBSSID is disabled.

Function ieee80211_stop() is modified to always bring down all the
non-transmitting interfaces first and the transmitting interface last.

Signed-off-by: John Crispin <john@phrozen.org>
Co-developed-by: Aloka Dixit <alokad@codeaurora.org>
Signed-off-by: Aloka Dixit <alokad@codeaurora.org>
Link: https://lore.kernel.org/r/20210916025437.29138-3-alokad@codeaurora.org


[slightly change logic to be more obvious]
Signed-off-by: Johannes Berg <johannes.berg@intel.com>