dmaengine: Fix return value for dma_request_chan() in case of failure

Commit 71723a96 ("dmaengine: Create symlinks between DMA channels and
slaves") changed the dma_request_chan() function flow in such a way that
it always returns EPROBE_DEFER in case of channels that cannot be found.
This break the operation of the devices which have optional DMA channels
as it puts their drivers in endless deferred probe loop. Fix this by
propagating the proper error value.

Fixes: 71723a96 ("dmaengine: Create symlinks between DMA channels and slaves")
Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://lore.kernel.org/r/20200130070834.17537-1-m.szyprowski@samsung.com
[vkoul: fix typo in patch title]
Signed-off-by: Vinod Koul <vkoul@kernel.org>

drivers/dma/dmaengine.c

@@ -759,7 +759,7 @@ struct dma_chan *dma_request_chan(struct device *dev, const char *name)
 	if (!IS_ERR_OR_NULL(chan))
 		goto found;
 
-	return ERR_PTR(-EPROBE_DEFER);
+	return chan ? chan : ERR_PTR(-EPROBE_DEFER);
 
 found:
 	chan->slave = dev;