ASoC: rt700: Add __maybe_unused to PM callbacks

ASoC: rt700: Add __maybe_unused to PM callbacks

Fix the compile warnings by adding __maybe_unused to PM callbacks:
  sound/soc/codecs/rt700-sdw.c:503:12: warning: ‘rt700_dev_resume’ defined but not used [-Wunused-function]
  sound/soc/codecs/rt700-sdw.c:489:12: warning: ‘rt700_dev_suspend’ defined but not used [-Wunused-function]

Fixes: 7d2a5f9ae41e ("ASoC: rt700: add rt700 codec driver")
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Link: https://lore.kernel.org/r/20200127192831.14057-3-tiwai@suse.de
Signed-off-by: Mark Brown <broonie@kernel.org>