ASoC: sta350: 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.

I just checked the code in commit 34d7c390
("ASoC: improve usage of gpiod API") and found that it should use
devm_gpiod_get_optional rather than devm_gpiod_get here.

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