mwl8k: don't overwrite mwl8k_vif::bssid until after disassociation

When disassociating, mac80211 zeroes vif->bss_info.bssid before
calling our ->bss_info_changed(), but we need the BSSID to remove the
hardware station database entry for our AP, so we can't clear our
local copy of the BSSID until after we've done that.

Signed-off-by: Lennert Buytenhek <buytenh@marvell.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>