Use the PCI device directly for dma accesses as non PCI device unlikely
support IOMMU and dma mappings.
Introduce and use helper routine to access DMA device.

Signed-off-by: Parav Pandit <parav@nvidia.com>
Reviewed-by: Vu Pham <vuhuong@nvidia.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>