ASoC: ak4641: Move register defines to main source

The ak4641 driver only has a few register defines. As they are only used in
the one main driver file there is not really a need to keep them in a
separate header.

Moving them to the main source file allows to remove the now empty header
file completely.

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