nvme/id-ns: do not try to get namespace id from non-block device.

nvme/id-ns: do not try to get namespace id from non-block device.

namespace id's default value is 0 and changed to 15 by get_nsid()
when device is not a block device.

With this change:
# ./nvme id-ns /dev/nvme0 -n 0
Error: requesting namespace-id from non-block device
NVMe Status:INVALID_NS(b) NSID:0
                               ^^
Without this change:
Missleading output when user passed 0 or 15 as namespace id.
# nvme id-ns /dev/nvme0
Error: requesting namespace-id from non-block device
NVMe Status:INVALID_NS(b) NSID:15
                               ^^
# nvme id-ns /dev/nvme0 -n 0
Error: requesting namespace-id from non-block device
NVMe Status:INVALID_NS(b) NSID:15
                               ^^
# nvme id-ns /dev/nvme0 -n 15
NVMe Status:INVALID_NS(b) NSID:15

Signed-off-by: Xiao Liang <xiliang@redhat.com>