spi/davinci: Fix DMA API usage in davinci

The driver uses NULL for dma_unmap_single instead of
the struct device that the API expects.

Signed-off-by: Shubhrajyoti D <shubhrajyoti@ti.com>
Tested-by: Akshay Shankarmurthy <akshay.s@ti.com>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>