ASoC: rt5645: Use devm_gpiod_get_optional for hp-detect

Since hp-detect is optional, use devm_gpiod_get_optional instead.
In additional, it should return error if devm_gpiod_get_optional fails.

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