net: inline rollback_registered()

rollback_registered() is a local helper, it's common for driver
code to call unregister_netdevice_queue(dev, NULL) when they
want to unregister netdevices under rtnl_lock. Inline
rollback_registered() and adjust the only remaining caller.

Reviewed-by: Edwin Peer <edwin.peer@broadcom.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>