ASoC: soc-generic-dmaengine-pcm: Use default config when none is given

ASoC: soc-generic-dmaengine-pcm: Use default config when none is given

snd_dmaengine_pcm_register() can be passed a NULL pointer for the config
which means that the we have to test for pcm->config being non NULL
before accessing it. Make the code more straight forward by providing a
default config when none is passed. With this pcm->config is never NULL
and we can skip all the if (pcm->config) tests.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>
Link: https://lore.kernel.org/r/20220502131335.2604158-1-s.hauer@pengutronix.de
Signed-off-by: Mark Brown <broonie@kernel.org>