soundwire: Fix duplicate stream state assignment

For a SoundWire stream it is expected that a Slave is added to the
stream before Master is added.

So, move the stream state to CONFIGURED after the first Slave is
added and remove the stream state assignment for Master add.
Along with these changes, add additional comments to explain the same.

Signed-off-by: Shreyas NC <shreyas.nc@intel.com>
Acked-by: Pierre-Louis Bossart <pierre-louis.bossart@linux.intel.com>
Signed-off-by: Vinod Koul <vkoul@kernel.org>