ASoC: core: Initialize err for snd_soc_put_volsw_sx

sound/soc/soc-core.c: In function ‘snd_soc_put_volsw_sx’:
sound/soc/soc-core.c:2600: warning: ‘err’ may be used uninitialized in this function

Signed-off-by: Brian Austin <brian.austin@cirrus.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>