ASoC: soc-core: return true, false in snd_soc_volsw_is_stereo()

Fix the following coccicheck warning:

include/sound/soc.h:1271:9-10: WARNING: return of 0/1 in function
'snd_soc_volsw_is_stereo' with return type bool

Signed-off-by: Jason Yan <yanaijie@huawei.com>
Link: https://lore.kernel.org/r/20200422071805.48793-1-yanaijie@huawei.com


Signed-off-by: Mark Brown <broonie@kernel.org>