USB: usb peripheral controller driver oops avoidance

I'm having problem with oopses when rebooting, if I modprobe g_serial
and rmmod g_serial and do a reboot I get an oops in device_shutdown().
The reason seems to be that usb_gadget_unregister_driver() doesn't do
enough cleanup.  With this at91_udc patch I don't get the oops.

Signed-off-by: Patrik Sevallius <patrik.sevallius@enea.com>
[ Same bug was in other peripheral controller drivers; fixed ]
Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>