spi: pic32: Use proper enum in dmaengine_prep_slave_rg

Clang warns when one enumerated type is converted implicitly to another:

drivers/spi/spi-pic32.c:323:8: warning: implicit conversion from
enumeration type 'enum dma_data_direction' to different enumeration type
'enum dma_transfer_direction' [-Wenum-conversion]
                                          DMA_FROM_DEVICE,
                                          ^~~~~~~~~~~~~~~
drivers/spi/spi-pic32.c:333:8: warning: implicit conversion from
enumeration type 'enum dma_data_direction' to different enumeration type
'enum dma_transfer_direction' [-Wenum-conversion]
                                          DMA_TO_DEVICE,
                                          ^~~~~~~~~~~~~
2 warnings generated.

Use the proper enums from dma_transfer_direction (DMA_FROM_DEVICE =
DMA_DEV_TO_MEM = 2, DMA_TO_DEVICE = DMA_MEM_TO_DEV = 1) to satify Clang.

Link: https://github.com/ClangBuiltLinux/linux/issues/159


Signed-off-by: Nathan Chancellor <natechancellor@gmail.com>
Signed-off-by: Mark Brown <broonie@kernel.org>