Fix the following gcc warning:

sound/soc/codecs/wm8990.c:1309:35: warning: ‘wm8990_regmap’ defined but
not used [-Wunused-const-variable=]
 static const struct regmap_config wm8990_regmap = {
                                   ^~~~~~~~~~~~~
sound/soc/codecs/wm8990.c:490:38: warning:
‘wm8990_dapm_rxvoice_controls’ defined but not used
[-Wunused-const-variable=]
 static const struct snd_kcontrol_new wm8990_dapm_rxvoice_controls[] = {
                                      ^~~~~~~~~~~~~~~~~~~~~~~~~~~~
sound/soc/codecs/wm8990.c:120:35: warning: ‘out_omix_tlv’ defined but
not used [-Wunused-const-variable=]
 static const DECLARE_TLV_DB_SCALE(out_omix_tlv, -600, 0, 0);
                                   ^
sound/soc/codecs/wm8990.c:112:35: warning: ‘rec_mix_tlv’ defined but not
used [-Wunused-const-variable=]
 static const DECLARE_TLV_DB_SCALE(rec_mix_tlv, -1500, 600, 0);
                                   ^
Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Jason Yan <yanaijie@huawei.com>
Link: https://lore.kernel.org/r/20200407082932.41511-4-yanaijie@huawei.comSigned-off-by: Mark Brown <broonie@kernel.org>