ASoC: rt715: Add __maybe_unused to PM callbacks

Fix the compile warnings by adding __maybe_unused to PM callbacks:
  sound/soc/codecs/rt715-sdw.c:566:12: warning: ‘rt715_dev_resume’ defined but not used [-Wunused-function]
  sound/soc/codecs/rt715-sdw.c:552:12: warning: ‘rt715_dev_suspend’ defined but not used [-Wunused-function]

Fixes: d1ede064 ("ASoC: rt715: add RT715 codec driver")
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Link: https://lore.kernel.org/r/20200127192831.14057-5-tiwai@suse.deSigned-off-by: Mark Brown <broonie@kernel.org>

sound/soc/codecs/rt715-sdw.c

@@ -549,7 +549,7 @@ static const struct sdw_device_id rt715_id[] = {
 };
 MODULE_DEVICE_TABLE(sdw, rt715_id);
 
-static int rt715_dev_suspend(struct device *dev)
+static int __maybe_unused rt715_dev_suspend(struct device *dev)
 {
 	struct rt715_priv *rt715 = dev_get_drvdata(dev);
 
@@ -563,7 +563,7 @@ static int rt715_dev_suspend(struct device *dev)
 
 #define RT715_PROBE_TIMEOUT 2000
 
-static int rt715_dev_resume(struct device *dev)
+static int __maybe_unused rt715_dev_resume(struct device *dev)
 {
 	struct sdw_slave *slave = dev_to_sdw_dev(dev);
 	struct rt715_priv *rt715 = dev_get_drvdata(dev);