when bringing up ethernet on ixp4xx, the driver may request
a firmware while hotplug2 waits for the netdev bringup to complete.
change the event processing loop to keep an event backlog to allow
unthrottled events to pass through while throttled events are being waited for
git-svn-id: svn://svn.openwrt.org/openwrt/trunk@12019 3c298f89-4303-0410-b956-a3cf2f4a3e73
