ASoC: fix compile breakage of the sh/siu driver

The sh/siu ASoC driver doesn't compile because of a function defined static in
the source and extern in a header. Remove the unneeded declaration in the
header.

Signed-off-by: Guennadi Liakhovetski <g.liakhovetski@gmx.de>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>