ASoC: soc-topology: cleanup cppcheck warning at soc_tplg_kcontrol_elems_load()

ASoC: soc-topology: cleanup cppcheck warning at soc_tplg_kcontrol_elems_load()

This patch cleanups below cppcheck warning.

sound/soc/soc-topology.c:1038:31: style: The scope of the variable 'control_hdr' can be reduced. [variableScope]
 struct snd_soc_tplg_ctl_hdr *control_hdr;
                              ^

Signed-off-by: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
Link: https://lore.kernel.org/r/87o8agwfkv.wl-kuninori.morimoto.gx@renesas.com
Signed-off-by: Mark Brown <broonie@kernel.org>