ASoC: tegra: mark runtime-pm functions as __maybe_unused

ASoC: tegra: mark runtime-pm functions as __maybe_unused

A reorganization of the driver source led to two of them causing
a compile time warning in some configurations:

tegra/tegra20_spdif.c:36:12: error: 'tegra20_spdif_runtime_resume' defined but not used [-Werror=unused-function]
   36 | static int tegra20_spdif_runtime_resume(struct device *dev)
      |            ^~~~~~~~~~~~~~~~~~~~~~~~~~~~
tegra/tegra20_spdif.c:27:12: error: 'tegra20_spdif_runtime_suspend' defined but not used [-Werror=unused-function]
   27 | static int tegra20_spdif_runtime_suspend(struct device *dev)
      |            ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~
tegra/tegra30_ahub.c:64:12: error: 'tegra30_ahub_runtime_resume' defined but not used [-Werror=unused-function]
   64 | static int tegra30_ahub_runtime_resume(struct device *dev)
      |            ^~~~~~~~~~~~~~~~~~~~~~~~~~~
tegra/tegra30_ahub.c:43:12: error: 'tegra30_ahub_runtime_suspend' defined but not used [-Werror=unused-function]
   43 | static int tegra30_ahub_runtime_suspend(struct device *dev)
      |            ^~~~~~~~~~~~~~~~~~~~~~~~~~~~

Mark these functions as __maybe_unused to avoid this kind of warning.

Fixes: b5571449e618 ("ASoC: tegra30: ahub: Remove handing of disabled runtime PM")
Fixes: c53b396f0dd4 ("ASoC: tegra20: spdif: Remove handing of disabled runtime PM")
Fixes: 80ec4a4cb36d ("ASoC: tegra20: i2s: Remove handing of disabled runtime PM")
Fixes: b5f6f781fcb2 ("ASoC: tegra30: i2s: Remove handing of disabled runtime PM")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Link: https://lore.kernel.org/r/20210422133418.1757893-1-arnd@kernel.org
Signed-off-by: Mark Brown <broonie@kernel.org>