nvme: fix an error code in nvme_init_subsystem()

"ret" should be a negative error code here, but it's either success or
possibly uninitialized.

Fixes: 32fd90c4 ("nvme: change locking for the per-subsystem controller list")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: Keith Busch <kbusch@kernel.org>
Signed-off-by: Sagi Grimberg <sagi@grimberg.me>