nvme-pci: merge the simple PRP and SGL setup into a common helper

nvme_setup_prp_simple and nvme_setup_sgl_simple share a lot of logic.
Merge them into a single helper that makes use of the previously added
use_sgl tristate.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Keith Busch <kbusch@kernel.org>
Link: https://lore.kernel.org/r/20250625113531.522027-5-hch@lst.de


Signed-off-by: Jens Axboe <axboe@kernel.dk>