ASoC: rt5682-sdw: fix 'defined but not used' pm functions

ASoC: rt5682-sdw: fix 'defined but not used' pm functions

Gcc reports the following warnings:

sound/soc/codecs/rt5682-sdw.c:286:12: warning: 'rt5682_dev_resume'
defined but not used [-Wunused-function]
 static int rt5682_dev_resume(struct device *dev)
            ^~~~~~~~~~~~~~~~~

sound/soc/codecs/rt5682-sdw.c:273:12: warning: 'rt5682_dev_suspend'
defined but not used [-Wunused-function]
 static int rt5682_dev_suspend(struct device *dev)
            ^~~~~~~~~~~~~~~~~~

Fix by adding maybe_unused as done for other SoundWire codecs

Fixes: 03f6fc6de9192f ('ASoC: rt5682: Add the soundwire support')
Reported-by: kbuild test robot <lkp@intel.com>
Signed-off-by: Pierre-Louis Bossart <pierre-louis.bossart@linux.intel.com>
Cc: Oder Chiou <oder_chiou@realtek.com>
Link: https://lore.kernel.org/r/20200310163509.14466-4-pierre-louis.bossart@linux.intel.com
Signed-off-by: Mark Brown <broonie@kernel.org>