Update the driver to prevent alsa-restore.service from failing when
reading data from /var/lib/alsa/asound.state at boot. Ensure that the
restoration of ALSA mixer configurations is skipped if substream->runtime
is NULL.

Fixes: 50291652 ("ASoC: atmel: mchp-pdmc: add PDMC driver")
Signed-off-by: Andrei Simion <andrei.simion@microchip.com>
Link: https://patch.msgid.link/20240924081237.50046-1-andrei.simion@microchip.comSigned-off-by: Mark Brown <broonie@kernel.org>