ASoC: dwc: i2s: Fix unused functions

A few newly added functions aren't built unless CONFIG_OF is set,
which result in the build failure due to defined-but-not-used errors.

Put "#ifdef CONFIG_OF" around those functions to suppress the build
error.

Fixes: 52ea7c05 ("ASoC: dwc: i2s: Add StarFive JH7110 SoC support")
Link: https://lore.kernel.org/r/20230828113537.27600-1-tiwai@suse.de


Acked-by: Mark Brown <broonie@kernel.org>
Signed-off-by: Takashi Iwai <tiwai@suse.de>