ASoC: cs35l36: Make some symbols static

Fixes the following sparse warnings:

sound/soc/codecs/cs35l36.c:135:20: warning:
 symbol 'cs35l36_reg' was not declared. Should it be static?
sound/soc/codecs/cs35l36.c:248:6: warning:
 symbol 'cs35l36_readable_reg' was not declared. Should it be static?
sound/soc/codecs/cs35l36.c:398:6: warning:
 symbol 'cs35l36_precious_reg' was not declared. Should it be static?
sound/soc/codecs/cs35l36.c:410:6: warning:
 symbol 'cs35l36_volatile_reg' was not declared. Should it be static?

Fixes: 6ba9dd6c ("ASoC: cs35l36: Add support for Cirrus CS35L36 Amplifier")
Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Acked-by: James Schulman <james.schulman@cirrus.com>
Signed-off-by: Mark Brown <broonie@kernel.org>