ASoC: cirrus: Add helper function for reading the device ID

Many of the older Cirrus devices share very similar code for reading the
device ID, and frequently this code is generating cppcheck warnings such
as:

sound/soc/codecs/cs42l42.c:1886:6: style: Variable 'ret' is reassigned
a value before the old one has been used. [redundantAssignment]
ret = regmap_read(cs42l42->regmap, CS42L42_DEVID_CD, &reg);

Add a small helper function that older Cirrus devices can use to read
the device ID, which should help correct these issues.

Reported-by: Pierre-Louis Bossart <pierre-louis.bossart@linux.intel.com>
Signed-off-by: Charles Keepax <ckeepax@opensource.cirrus.com>
Link: https://lore.kernel.org/r/20210510131357.17170-2-ckeepax@opensource.cirrus.com
Signed-off-by: Mark Brown <broonie@kernel.org>