ASoC: dwc: Get IRQ optionally

The IRQ is explicitly optional, so use platform_get_irq_optional() and
avoid platform_get_irq() logging a spurious error when trying to use the
thing in DMA mode.

Signed-off-by: Robin Murphy <robin.murphy@arm.com>
Link: https://lore.kernel.org/r/c857f334e3c9e651e088b675b3938cb5f798b133.1629906123.git.robin.murphy@arm.com


Signed-off-by: Mark Brown <broonie@kernel.org>