ASoC: codecs: nau8825: fix kernel-doc

ASoC: codecs: nau8825: fix kernel-doc

v5.12-rc1 flags new warnings with make W=1, fix missing or broken
function descriptors.

sound/soc/codecs/nau8825.c:298: warning: wrong kernel-doc identifier
on line:
 * Ramp up the headphone volume change gradually to target level.

sound/soc/codecs/nau8825.c:358: warning: expecting prototype for This
func(). Prototype was for nau8825_intlog10_dec3() instead

sound/soc/codecs/nau8825.c:411: warning: wrong kernel-doc identifier
on line:
 * computes cross talk suppression sidetone gain.

Signed-off-by: Pierre-Louis Bossart <pierre-louis.bossart@linux.intel.com>
Link: https://lore.kernel.org/r/20210301174639.117017-1-pierre-louis.bossart@linux.intel.com
Signed-off-by: Mark Brown <broonie@kernel.org>