dmaengine: ti: omap-dma: Skip pointless cpu_pm context restore on errors

There's no need to restore DMA context on CPU_CLUSTER_PM_ENTER_FAILED as
the DMA context won't be lost on errors.

Note that this does not cause invalid context restore as we already check
for busy DMA with omap_dma_busy() in CPU_CLUSTER_PM_ENTER, and block any
deeper idle states for the SoC by returning NOTIFY_BAD if busy.

If other drivers block deeper idle states with cpu_pm, we now just do a
pointless restore, but only if dma was not busy on CPU_CLUSTER_PM_ENTER.

Let's update the CPU_CLUSTER_PM_ENTER_FAILED handling for correctness,
and add a comment.

Cc: Aaro Koskinen <aaro.koskinen@iki.fi>
Cc: Adam Ford <aford173@gmail.com>
Cc: Andreas Kemnade <andreas@kemnade.info>
Cc: Peter Ujfalusi <peter.ujfalusi@gmail.com>
Signed-off-by: NTony Lindgren <tony@atomide.com>
Acked-by: NPeter Ujfalusi <peter.ujfalusi@gmail.com>
Link: https://lore.kernel.org/r/20210518074347.16908-1-tony@atomide.comSigned-off-by: NVinod Koul <vkoul@kernel.org>

drivers/dma/ti/omap-dma.c

@@ -1608,7 +1608,8 @@ static int omap_dma_context_notifier(struct notifier_block *nb,
 			return NOTIFY_BAD;
 		omap_dma_context_save(od);
 		break;
-	case CPU_CLUSTER_PM_ENTER_FAILED:
+	case CPU_CLUSTER_PM_ENTER_FAILED:	/* No need to restore context */
+		break;
 	case CPU_CLUSTER_PM_EXIT:
 		omap_dma_context_restore(od);
 		break;