iwlwifi: revamp bss_info_changed

My earlier patch,
	"mac80211: unify config_interface and bss_info_changed"
introduced a bug in iwlwifi where it will do some things
incorrectly now when reassociating.

Revamp iwl_bss_info_changed to fix that issue and make it
easier to read. Also, while at it, add comments about things
that it should do but currently doesn't.

Finally, also improve the locking in the function.

Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: Reinette Chatre <reinette.chatre@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>