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

sound/soc/intel/atom/sst-atom-controls.c

@@ -723,7 +723,7 @@ SST_SBA_DECLARE_MIX_CONTROLS(sst_mix_pcm2_controls);
 SST_SBA_DECLARE_MIX_CONTROLS(sst_mix_sprot_l0_controls);
 SST_SBA_DECLARE_MIX_CONTROLS(sst_mix_media_l1_controls);
 SST_SBA_DECLARE_MIX_CONTROLS(sst_mix_media_l2_controls);
-SST_SBA_DECLARE_MIX_CONTROLS(sst_mix_voip_controls);
+SST_SBA_DECLARE_MIX_CONTROLS(__maybe_unused sst_mix_voip_controls);
 SST_SBA_DECLARE_MIX_CONTROLS(sst_mix_codec0_controls);
 SST_SBA_DECLARE_MIX_CONTROLS(sst_mix_codec1_controls);
 SST_SBA_DECLARE_MIX_CONTROLS(sst_mix_modem_controls);