nvme-pci: tidy up nvme_map_data

Remove two pointless local variables, remove ret assignment that is
never used, move the use_sgl initialization closer to where it is used.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Keith Busch <keith.busch@intel.com>
Reviewed-by: Sagi Grimberg <sagi@grimberg.me>
Reviewed-by: Chaitanya Kulkarni <chaitanya.kulkarni@wdc.com>