spi: spi-atmel: Use correct enum for DMA transfer direction

Use enum dma_transfer_direction as required by the functions
dmaengine_prep_slave_(sg|single)() instead of enum dma_data_direction.
This won't change behavior in practice as the enum values are
equivalent.

This fixes two warnings when building with clang:
  drivers/spi/spi-atmel.c:771:12: warning: implicit conversion from enumeration
      type 'enum dma_data_direction' to different enumeration type
      'enum dma_transfer_direction'
      [-Wenum-conversion]
                                                     DMA_FROM_DEVICE,
                                                     ^~~~~~~~~~~~~~~
  ...

Signed-off-by: Stefan Agner <stefan@agner.ch>
Acked-by: Nicolas Ferre <nicolas.ferre@microchip.com>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Mark Brown <broonie@kernel.org>