nvme: send Identify with CNS 06h only to I/O controllers

Identify CNS 06h (I/O Command Set Specific Identify Controller data
structure) is supported only on i/o controllers.

But nvme_init_non_mdts_limits() currently invokes this on all
controllers.  Correct this by ensuring this is sent to I/O
controllers only.

Signed-off-by: Martin George <marting@netapp.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>