ASoC: Intel: sof_sdw_rt1308: add extra check on init

Apply same test as for other amplifiers - in case we enable feedback
one day.

Signed-off-by: Pierre-Louis Bossart <pierre-louis.bossart@linux.intel.com>
Reviewed-by: Bard Liao <yung-chuan.liao@linux.intel.com>
Link: https://lore.kernel.org/r/20201002211902.287692-2-pierre-louis.bossart@linux.intel.com


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