nvme-pci: don't fall back to a 32-bit DMA mask

Since Linux 5.0 drivers can safely set the largest DMA mask supported
by the device, and don't need fallbacks to work around the dma mapping
implementations.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Sagi Grimberg <sagi@grimberg.me>