br2684: fix scheduling while atomic

You can't call atomic_notifier_chain_unregister() while in atomic context.

Fix, call un/register_atmdevice_notifier in module __init and __exit.

Bug report:
http://comments.gmane.org/gmane.linux.network/172603



Reported-by: Mikko Vinni <mmvinni@yahoo.com>
Tested-by: Mikko Vinni <mmvinni@yahoo.com>
Signed-off-by: Karl Hiramoto <karl@hiramoto.org>
Signed-off-by: David S. Miller <davem@davemloft.net>