ASoC: rt1308-sdw: add set_tdm_slot() support

Add ability to select which of the channels is used, or both, in case
two RT1308 amplifiers are located on the same link.

Signed-off-by: Pierre-Louis Bossart <pierre-louis.bossart@linux.intel.com>
Link: https://lore.kernel.org/r/20200325212905.28145-2-pierre-louis.bossart@linux.intel.com


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