nvme-pci: fix psdt field for single segment sgls

The shortcut for single segment SGL requests did not set the PSDT field
to mark the request as using SGLs.

Fixes: 29791057 ("nvme-pci: optimize mapping single segment requests using SGLs")
Signed-off-by: Klaus Birkelund Jensen <klaus.jensen@cnexlabs.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>