wifi: mac80211: replace link_id with link_conf in start/stop_ap()

When calling start/stop_ap(), mac80211 already has a protected
link_conf pointer. Pass it to the driver, so it shouldn't
handle RCU protection.

Signed-off-by: Gregory Greenman <gregory.greenman@intel.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>