nvme: Add constants for the PSDT field in cdw0

This is not used in nvme(4) but is used in NVMe over Fabrics
transports which use SGLs to describe buffers instead of PRPs.

While here, adjust the shift value for the FUSE field to be relative
to the 'fuse' member of 'struct nvme_command'.

Reviewed by: imp
Sponsored by: Chelsio Communications
Differential Revision: https://reviews.freebsd.org/D44443