ASoC: loongson: make loongson-i2s.o a separate module

An object file should not be linked into multiple modules and/or
vmlinux:

scripts/Makefile.build:221: /home/arnd/arm-soc/sound/soc/loongson/Makefile: loongson_i2s.o is added to multiple modules: snd-soc-loongson-i2s-pci snd-soc-loongson-i2s-plat

Change this one to make it a library module with two exported symbols
that will work in any configuration.

Fixes: ba4c5fad ("ASoC: loongson: Add I2S controller driver as platform device")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Link: https://patch.msgid.link/20241015150958.2294155-1-arnd@kernel.org


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