ASoC: rt5514: mark PM functions as __maybe_unused

The new functions are only used when CONFIG_PM is enabled,
leading to a harmless warning:

sound/soc/codecs/rt5514-spi.c:474:12: error: 'rt5514_resume' defined but not used [-Werror=unused-function]
sound/soc/codecs/rt5514-spi.c:464:12: error: 'rt5514_suspend' defined but not used [-Werror=unused-function]

This marks them as __maybe_unused to make the build silent
again.

Fixes: 58f1c07d ("ASoC: rt5514: Voice wakeup support.")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Mark Brown <broonie@kernel.org>