dmaengine: ep93xx: Return proper enum in ep93xx_dma_chan_direction

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

In file included from drivers/dma/ep93xx_dma.c:30:
./include/linux/platform_data/dma-ep93xx.h:88:10: warning: implicit
conversion from enumeration type 'enum dma_data_direction' to different
enumeration type 'enum dma_transfer_direction' [-Wenum-conversion]
                return DMA_NONE;
                ~~~~~~ ^~~~~~~~
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>