firmware: cs_dsp: Add offset to cs_dsp read/write

Provide a mechanism to access only part of a control through the cs_dsp
interface.

Signed-off-by: Charles Keepax <ckeepax@opensource.cirrus.com>
Link: https://lore.kernel.org/r/20211117132300.1290-9-ckeepax@opensource.cirrus.com


Signed-off-by: Mark Brown <broonie@kernel.org>