dmaengine: fix incorrect kernel-doc comment for struct dma_slave_config

The 'direction' member of 'struct dma_slave_config' is of data
type 'enum dma_transfer_direction', so update the kernel-doc comment
for 'struct dma_slave_config' to refer to appropriate values.
Signed-off-by: Alexander Popov <a13xp0p0v88@gmail.com>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>

include/linux/dmaengine.h

@@ -305,9 +305,8 @@ enum dma_slave_buswidth {
 /**
  * struct dma_slave_config - dma slave channel runtime config
  * @direction: whether the data shall go in or out on this slave
- * channel, right now. DMA_TO_DEVICE and DMA_FROM_DEVICE are
- * legal values, DMA_BIDIRECTIONAL is not acceptable since we
- * need to differentiate source and target addresses.
+ * channel, right now. DMA_MEM_TO_DEV and DMA_DEV_TO_MEM are
+ * legal values.
  * @src_addr: this is the physical address where DMA slave data
  * should be read (RX), if the source is memory this argument is
  * ignored.