ASoC: rockchip: use __maybe_unused to hide st_irq_syscfg_resume

The rockchip spdif driver uses SIMPLE_DEV_PM_OPS to conditionally
set its power management functions, but we get a warning
about rk_spdif_runtime_resume being unused when CONFIG_PM is not
set:

sound/soc/rockchip/rockchip_spdif.c:67:12: error: 'rk_spdif_runtime_resume' defined but not used [-Werror=unused-function]

This adds a __maybe_unused annotation so the compiler knows
it can silently drop it instead of warning.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Mark Brown <broonie@kernel.org>