ASoC: Intel: atom: fix 'defined but not used' warning

Fix W=1 warning. The VOIP controls were not used in the mainline but
in special versions of Android. Keep and use __maybe_used to make
warning go away.

Signed-off-by: Pierre-Louis Bossart <pierre-louis.bossart@linux.intel.com>
Link: https://lore.kernel.org/r/20200701183716.83314-3-pierre-louis.bossart@linux.intel.com
Signed-off-by: Mark Brown <broonie@kernel.org>