dmaengine: pl330: Really fix choppy sound because of wrong residue calculation

When pl330 driver was used during sound playback, after some time or
after a number of plays the sound became choppy or totally noisy. For
example on Odroid XU3 board the first four executions of aplay with
small WAVE worked fine, but fifth was unrecognizable with errors:
	$ aplay /usr/share/sounds/alsa/Front_Right.wava
	underrun!!! (at least 0.095 ms long)

Issue was caused by wrong residue reported by pl330 driver to
pcm_dmaengine for its cyclic dma transfers.

The pl330_tx_status(), residue reporting function, used a "last" flag in
a descriptor to indicate that there is no more data to send.

The pl330_tx_submit() iterated over descriptors trying to remove this
flag from them and then mark last descriptor as "last".  However when
iterating it actually removed the flag not from descriptors but always
from last of it (and then reset it). Thus effectively once some
descriptor was marked as last, then it stayed like this forever causing
residue to be reported too low.

Signed-off-by: Krzysztof Kozlowski <k.kozlowski.k@gmail.com>
Fixes: aee4d1fac8 ("dmaengine: pl330: improve pl330_tx_status() function")
Cc: <stable@vger.kernel.org>
Reported-by: gabriel@unseen.is
Suggested-by: Marek Szyprowski <m.szyprowski@samsung.com>
Tested-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>

drivers/dma/pl330.c

@@ -2328,7 +2328,7 @@ static dma_cookie_t pl330_tx_submit(struct dma_async_tx_descriptor *tx)
 			desc->txd.callback = last->txd.callback;
 			desc->txd.callback_param = last->txd.callback_param;
 		}
-		last->last = false;
+		desc->last = false;
 
 		dma_cookie_assign(&desc->txd);