spi/davinci: Use correct length parameter to dma_map_single calls

The davinci spi driver provides an option to use DMA transfers for
data.  In the dma_map_single() call, the driver is passing the
number of words to be transfered for the mapping size.  It should
be the number of bytes.

Signed-off-by: Michael Williamson <michael.williamson@criticallink.com>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>