ASoC: fsl_aud2htx: mark PM functions as __maybe_unused

ASoC: fsl_aud2htx: mark PM functions as __maybe_unused

When CONFIG_PM is disabled, we get a warning for unused functions:

sound/soc/fsl/fsl_aud2htx.c:261:12: error: unused function 'fsl_aud2htx_runtime_suspend' [-Werror,-Wunused-function]
static int fsl_aud2htx_runtime_suspend(struct device *dev)
sound/soc/fsl/fsl_aud2htx.c:271:12: error: unused function 'fsl_aud2htx_runtime_resume' [-Werror,-Wunused-function]
static int fsl_aud2htx_runtime_resume(struct device *dev)

Mark these as __maybe_unused to avoid the warning without adding
an #ifdef.

Fixes: 8a24c834c053 ("ASoC: fsl_aud2htx: Add aud2htx module driver")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Nicolin Chen <nicoleotsuka@gmail.com>
Link: https://lore.kernel.org/r/20201203222900.1042578-1-arnd@kernel.org
Signed-off-by: Mark Brown <broonie@kernel.org>