mac80211: clean up association better in suspend

When suspending, bss_info_changed() is called to
disable beacons, but managed mode interfaces are
simply removed (bss_info_changed() is called with
"no change" only). This can lead to problems.

To fix this and copy the BSS configuration, clear
it during suspend and restore it on resume.

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