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.de
Signed-off-by: Mark Brown <broonie@kernel.org>