ASoC: codecs: rk3308: fix "defined but not used" warning on !OF

Building with CONFIG_OF=n triggers:

  warning: 'rk3308_codec_of_match' defined but not used [-Wunused-const-variable=]
  warning: unused variable 'rk3308_codec_of_match' [-Wunused-const-variable]

Even though OF is needed for probing, fix by declaring as __maybe_unused to
still allow building on non-OF configurations for build testing.

Fixes: 9fdd7b45 ("arm64: defconfig: enable Rockchip RK3308 internal audio codec driver")
Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202403271905.BYbGJiPi-lkp@intel.com/
Closes: https://lore.kernel.org/oe-kbuild-all/202403271907.0z0uuG5I-lkp@intel.com/Signed-off-by: Luca Ceresoli <luca.ceresoli@bootlin.com>
Link: https://msgid.link/r/20240403-rk3308-audio-codec-fix-warning-v2-1-816bae4c1dc5@bootlin.comSigned-off-by: Mark Brown <broonie@kernel.org>

sound/soc/codecs/rk3308_codec.c

@@ -953,7 +953,7 @@ static int rk3308_codec_platform_probe(struct platform_device *pdev)
 	return 0;
 }
 
-static const struct of_device_id rk3308_codec_of_match[] = {
+static const struct of_device_id __maybe_unused rk3308_codec_of_match[] = {
 	{ .compatible = "rockchip,rk3308-codec", },
 	{},
 };