soundwire: slave: add helper to extract slave ID

Simplify the loop with a helper. The only functionality change is that
we continue the loop even with an ACPI error.

Follow-up patches will build on this change.

Signed-off-by: Pierre-Louis Bossart <pierre-louis.bossart@linux.intel.com>
Link: https://lore.kernel.org/r/20191022234808.17432-3-pierre-louis.bossart@linux.intel.com
Signed-off-by: Vinod Koul <vkoul@kernel.org>