spi: spi-davinci: Fix direction in dma_map_single()

Commit 048177ce3b3962852fd34a7e04938959271c7e70 (spi: spi-davinci:
convert to DMA engine API) introduced a regression: dma_map_single()
is called with direction DMA_FROM_DEVICE for rx and for tx.

Signed-off-by: Christian Eggers <ceggers@gmx.de>
Acked-by: Matt Porter <mporter@ti.com>
Signed-off-by: Mark Brown <broonie@linaro.org>
Cc: stable@vger.kernel.org # v3.7.x+