dmaengine: clarify the meaning of the DMA_CTRL_ACK flag

DMA_CTRL_ACK's description applies to its clear state, not to its set state.
Signed-off-by: NGuennadi Liakhovetski <g.liakhovetski@gmx.de>
Signed-off-by: NDan Williams <dan.j.williams@intel.com>

include/linux/dmaengine.h

@@ -74,7 +74,7 @@ enum dma_transaction_type {
  *  control completion, and communicate status.
  * @DMA_PREP_INTERRUPT - trigger an interrupt (callback) upon completion of
  *  this transaction
- * @DMA_CTRL_ACK - the descriptor cannot be reused until the client
+ * @DMA_CTRL_ACK - if clear, the descriptor cannot be reused until the client
  *  acknowledges receipt, i.e. has has a chance to establish any dependency
  *  chains
  * @DMA_COMPL_SKIP_SRC_UNMAP - set to disable dma-unmapping the source buffer(s)