nvme-pci: remove useless assignment in nvme_pci_setup_prps

If prp_list is NULL, nvme_unmap_sg will be performed, and the assignment
to first_dma is meaningless, so remove it.

Signed-off-by: Liu Song <liusong@linux.alibaba.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>