soundwire: bus: check first if Slaves become UNATTACHED

Before checking for the presence of Device0, we first need to clean-up
the internal state of Slaves that are no longer attached.

Signed-off-by: Pierre-Louis Bossart <pierre-louis.bossart@linux.intel.com>
Link: https://lore.kernel.org/r/20200110215731.30747-7-pierre-louis.bossart@linux.intel.com


Signed-off-by: Vinod Koul <vkoul@kernel.org>