dmaengine: Remove unused define for dma_request_slave_channel_reason()

No users left in the kernel, it can be removed.

Signed-off-by: Peter Ujfalusi <peter.ujfalusi@ti.com>
Link: https://lore.kernel.org/r/20200828084141.14902-1-peter.ujfalusi@ti.com


Signed-off-by: Vinod Koul <vkoul@kernel.org>