ASoC: SigmaDSP: Move private structs and functions to C file

Move the structs and functions only used by SigmaDSP firmware loader itself
from the header to the C file.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Acked-by: Mike Frysinger <vapier@gentoo.org>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>