Use enum dma_transfer_direction as required by dmaengine_prep_slave_sg
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-rspi.c:538:26: warning: implicit conversion from enumeration
      type 'enum dma_data_direction' to different enumeration type
      'enum dma_transfer_direction' [-Wenum-conversion]
                                        rx->sgl, rx->nents, DMA_FROM_DEVICE,
                                                            ^~~~~~~~~~~~~~~
  drivers/spi/spi-rspi.c:558:26: warning: implicit conversion from enumeration
      type 'enum dma_data_direction' to different enumeration type
      'enum dma_transfer_direction' [-Wenum-conversion]
                                        tx->sgl, tx->nents, DMA_TO_DEVICE,
                                                            ^~~~~~~~~~~~~

Signed-off-by: Stefan Agner <stefan@agner.ch>
Signed-off-by: Mark Brown <broonie@kernel.org>