ASoC: stac9766: Move register defines to main source file

The stac9766 driver has a header file that defines 3 register locations.
Move these to the main source file since it is not really worth it having a
separate file for them. The header file is now empty and can be removed.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Mark Brown <broonie@kernel.org>