ASoC: core: Remove unused variable 'min'

commit 4183eed2 (ASoC: core: Add signed multi register control) introduced
the variable 'min',but it is not used.

Remove it to fix the following build warning:

sound/soc/soc-core.c: In function 'snd_soc_put_xr_sx':
sound/soc/soc-core.c:2990: warning: unused variable 'min'

Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>