ASoC: dmaengine: Improve of_node test in dmaengine_pcm_request_chan_of()

Currently when of_node of the "PCM" device is null
dmaengine_pcm_request_chan_of() function will bail out, including cases
when custom DMA device is intended to be used.  To have the channels
properly requested when custom DMA device is provided extend the of_node
test to also consider dma_dev->of_node.

Signed-off-by: Sylwester Nawrocki <s.nawrocki@samsung.com>
Acked-by: Krzysztof Kozlowski <krzk@kernel.org>
Signed-off-by: Mark Brown <broonie@kernel.org>