ASoC: dwc: Fix PIO mode initialization

We can no longer rely on the return value of
devm_snd_dmaengine_pcm_register(...) to check if the DMA
handle is declared in the DT.

Previously this check activated PIO mode but currently
dma_request_chan returns either a valid channel or -EPROBE_DEFER.

In order to activate PIO mode check instead if the interrupt
line is declared. This reflects better what is documented in
the DT bindings (see Documentation/devicetree/bindings/sound/
designware-i2s.txt).

Also, initialize use_pio variable which was never being set
causing PIO mode to never work.

Signed-off-by: Jose Abreu <joabreu@synopsys.com>
Signed-off-by: Mark Brown <broonie@kernel.org>