spi: cs42l43: Refactor accessing the SDCA extension properties

Refactor accessing the SDCA extension properties to make it easier to
access multiple properties to assist with future features. Return the
node itself and allow the caller to read the actual properties.

Signed-off-by: Charles Keepax <ckeepax@opensource.cirrus.com>
Link: https://patch.msgid.link/20240619121703.3411989-1-ckeepax@opensource.cirrus.com


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