nvme: requeue aen after firmware activation

The driver prevents async event work while handling a processing paused
event, but someone needs to restart it after the controller returns to a
live state.

Link: https://bugzilla.kernel.org/show_bug.cgi?id=216400
Signed-off-by: Keith Busch <kbusch@kernel.org>
Signed-off-by: Christoph Hellwig <hch@lst.de>