nvme: return kernel error codes for admin queue connect

nvmf_connect_admin_queue returns NVMe error status codes and kernel
error codes. This mixes the different domains which makes maintainability
difficult.

Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Sagi Grimberg <sagi@grimberg.me>
Signed-off-by: Hannes Reinecke <hare@suse.de>
Signed-off-by: Daniel Wagner <dwagner@suse.de>
Signed-off-by: Keith Busch <kbusch@kernel.org>