Fix the following gcc warning:

sound/soc/codecs/wm8900.c:449:38: warning:
‘wm8900_dapm_routput2_control’ defined but not used
[-Wunused-const-variable=]
 static const struct snd_kcontrol_new wm8900_dapm_routput2_control =
                                      ^~~~~~~~~~~~~~~~~~~~~~~~~~~~
sound/soc/codecs/wm8900.c:446:38: warning:
‘wm8900_dapm_loutput2_control’ defined but not used
[-Wunused-const-variable=]
 static const struct snd_kcontrol_new wm8900_dapm_loutput2_control =
                                      ^~~~~~~~~~~~~~~~~~~~~~~~~~~~
Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Jason Yan <yanaijie@huawei.com>
Link: https://lore.kernel.org/r/20200407082932.41511-5-yanaijie@huawei.comSigned-off-by: Mark Brown <broonie@kernel.org>