iwlwifi: mvm: fix AP/GO mode station removal

When stations are removed while packets are in the queue,
we drain the queues first, and then remove the stations.
If this happens in AP mode while the interface is removed
the MAC context might be removed from the firmware before
we removed the station(s), resulting in a SYSASSERT 3421.
This is because we remove the MAC context from the FW in
stop_ap(), but only flush the station drain work later in
remove_interface().

Refactor the code a bit to have a common MAC context
removal preparation first to solve this.

Reviewed-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>