mac80211: assign bss_conf.bssid only once

Instead of checking every time bss_info_changed is called,
assign the pointer once depending on the interface type
and then leave it untouched until the interface type is
changed. This makes the ieee80211_bss_info_change_notify()
now a simple wrapper to call the driver only.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>