soundwire: bus: fix race condition with initialization_complete signaling

Waiting for the enumeration to be complete may not be enough for a
Slave driver, there is a possible race condition between resume
operations and initializations handled in an interrupt thread, which
can results in settings not being fully restored after system or
pm_runtime resume.

This patch builds on the changes added for enumeration_complete,
init_completion() is called when the Slave device becomes UNATTACHED,
as done with enumeration_complete.

The difference with the enumeration_complete case is that complete()
is signaled after the Slave device is fully initialized after the
.update_status() callback is called.

A Slave device driver can decide to wait on either of the two
complete() cases, depending on its initialization code and
requirements.

Signed-off-by: Rander Wang <rander.wang@linux.intel.com>
Signed-off-by: Pierre-Louis Bossart <pierre-louis.bossart@linux.intel.com>
Link: https://lore.kernel.org/r/20200115000844.14695-4-pierre-louis.bossart@linux.intel.com
Signed-off-by: Vinod Koul <vkoul@kernel.org>