ASoC: SDCA: Add a helper to get the SoundWire port number

Add a helper function to extract the SoundWire hardware port number
from the SDCA DataPort Selector Control. Typically this would be
called from hw_params() and used to call sdw_stream_add_slave().

Signed-off-by: Charles Keepax <ckeepax@opensource.cirrus.com>
Reviewed-by: Pierre-Louis Bossart <pierre-louis.bossart@linux.dev>
Link: https://patch.msgid.link/20250707124155.2596744-7-ckeepax@opensource.cirrus.com


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