dmaengine: ep93xx: update the driver comments

driver comment refers to DMA_SLAVE_CONFIG which needs to be updated to
.device_config

Signed-off-by: Vinod Koul <vinod.koul@intel.com>
diff --git a/drivers/dma/ep93xx_dma.c b/drivers/dma/ep93xx_dma.c
index a8bcbb5..24e5290 100644
--- a/drivers/dma/ep93xx_dma.c
+++ b/drivers/dma/ep93xx_dma.c
@@ -144,7 +144,7 @@
  * @queue: pending descriptors which are handled next
  * @free_list: list of free descriptors which can be used
  * @runtime_addr: physical address currently used as dest/src (M2M only). This
- *                is set via %DMA_SLAVE_CONFIG before slave operation is
+ *                is set via .device_config before slave operation is
  *                prepared
  * @runtime_ctrl: M2M runtime values for the control register.
  *