dmaengine: timb_dma: Use proper enum in td_prep_slave_sg

Clang warns when implicitly converting from one enumerated type to
another. Avoid this by using the equivalent value from the expected
type.

drivers/dma/timb_dma.c:548:27: warning: implicit conversion from
enumeration type 'enum dma_transfer_direction' to different enumeration
type 'enum dma_data_direction' [-Wenum-conversion]
                td_desc->desc_list_len, DMA_MEM_TO_DEV);
                                        ^~~~~~~~~~~~~~
1 warning generated.

Reported-by: Nick Desaulniers <ndesaulniers@google.com>
Signed-off-by: Nathan Chancellor <natechancellor@gmail.com>
Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>
Signed-off-by: Vinod Koul <vkoul@kernel.org>