ASoC: soc-ops: cleanup cppcheck warning at snd_soc_limit_volume()

This patch cleanups below cppcheck warning.

sound/soc/soc-ops.c:576:28: style: The scope of the variable 'mc' can be reduced. [variableScope]
 struct soc_mixer_control *mc;
                           ^

Signed-off-by: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
Link: https://lore.kernel.org/r/8735rrw29q.wl-kuninori.morimoto.gx@renesas.com


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