ASoC: tegra: tegra210_dmic: Fix compile warning with CONFIG_PM=n

Fix trivial compile warnings wrt unused functions by adding
__maybe_unused prefix:
  sound/soc/tegra/tegra210_dmic.c:43:12: warning: 'tegra210_dmic_runtime_suspend' defined but not used [-Wunused-function]
  sound/soc/tegra/tegra210_dmic.c:55:12: warning: 'tegra210_dmic_runtime_resume' defined but not used [-Wunused-function]

Fixes: 8c8ff982 ("ASoC: tegra: Add Tegra210 based DMIC driver")
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Link: https://lore.kernel.org/r/20200803141850.23713-5-tiwai@suse.deSigned-off-by: Mark Brown <broonie@kernel.org>

sound/soc/tegra/tegra210_dmic.c

@@ -40,7 +40,7 @@ static const struct reg_default tegra210_dmic_reg_defaults[] = {
 	{ TEGRA210_DMIC_LP_BIQUAD_1_COEF_4, 0x0 },
 };
 
-static int tegra210_dmic_runtime_suspend(struct device *dev)
+static int __maybe_unused tegra210_dmic_runtime_suspend(struct device *dev)
 {
 	struct tegra210_dmic *dmic = dev_get_drvdata(dev);
 
@@ -52,7 +52,7 @@ static int tegra210_dmic_runtime_suspend(struct device *dev)
 	return 0;
 }
 
-static int tegra210_dmic_runtime_resume(struct device *dev)
+static int __maybe_unused tegra210_dmic_runtime_resume(struct device *dev)
 {
 	struct tegra210_dmic *dmic = dev_get_drvdata(dev);
 	int err;