iwlwifi: mvm: BT Coex fix another NULL pointer dereference

iwlwifi: mvm: BT Coex fix another NULL pointer dereference

This patch is very similar to a previous fix: 22cba0c0852f

When we disassociate, mac80211 removes the station and
then, it sets the bss it unsets the assoc bool in bss_info.

Since the firwmware wants it the opposite (first set the
MAC context as unassoc, and only then, remove the STA of
the API), we have a small period of time in which the STA
in firmware doesn't have a valid ieee80211_sta pointer.
During that time, iwl_mvm_vif->ap_sta_id, is still set
to the STA in firmware that represent the AP.

Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>