nvme: fix NULL pointer dereference in nvme_init_subsystem

When using nvme-pci driver the nvmf_ctrl_options is NULL.
There is no need to check for discovery_nqn flag at non-fabrics controller.

Fixes: 181303d0 ("nvme-fabrics: allow duplicate connections to the discovery controller")
Signed-off-by: Israel Rukshin <israelr@mellanox.com>
Reviewed-by: Max Gurtovoy <maxg@mellanox.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>