ASoC: loongson: fix unused PM function warning

ASoC: loongson: fix unused PM function warning

Build testing without CONFIG_PM_SLEEP causes a warning:

sound/soc/loongson/loongson_i2s.c:246:12: error: unused function 'i2s_suspend' [-Werror,-Wunused-function]
sound/soc/loongson/loongson_i2s.c:255:12: error: unused function 'i2s_resume' [-Werror,-Wunused-function]

Use the modern SYSTEM_SLEEP_PM_OPS() instead of the old one to avoid this.

Fixes: d24028606e764 ("ASoC: loongson: Add Loongson ASoC Sound Card Support")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Link: https://lore.kernel.org/r/20230616090156.2347850-2-arnd@kernel.org
Signed-off-by: Mark Brown <broonie@kernel.org>