ASoC: sta32x: Use devm_gpiod_get_optional at appropriate place

devm_gpiod_get_optional() is equivalent to devm_gpiod_get(), except that
when no GPIO was assigned to the requested function it will return NULL.
This is convenient for drivers that need to handle optional GPIOs.

Signed-off-by: Axel Lin <axel.lin@ingics.com>
Signed-off-by: Mark Brown <broonie@kernel.org>