ASoC: rsnd: mark PM functions __maybe_unused

The suspend/resume callbacks are now optional, leading to a warning
when they are unused:

sound/soc/sh/rcar/core.c:1548:12: error: 'rsnd_resume' defined but not used [-Werror=unused-function]
 static int rsnd_resume(struct device *dev)
            ^~~~~~~~~~~
sound/soc/sh/rcar/core.c:1539:12: error: 'rsnd_suspend' defined but not used [-Werror=unused-function]
 static int rsnd_suspend(struct device *dev)

This marks the as __maybe_unused to avoid the warning.

Fixes: f8a9a29c ("ASoC: rsnd: set pm_ops in hibernate-compatible way")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Mark Brown <broonie@kernel.org>