soundwire: stream: Add missing clear of alloc_slave_rt

soundwire: stream: Add missing clear of alloc_slave_rt

The current path that skips allocating the slave runtime does not clear
the alloc_slave_rt flag, this is clearly incorrect. Add the missing
clear, so the runtime won't be erroneously cleaned up.

Fixes: f3016b891c8c ("soundwire: stream: sdw_stream_add_ functions can be called multiple times")
Reviewed-by: Pierre-Louis Bossart <pierre-louis.bossart@linux.intel.com>
Signed-off-by: Charles Keepax <ckeepax@opensource.cirrus.com>
Link: https://lore.kernel.org/r/20230602101140.2040141-1-ckeepax@opensource.cirrus.com
Signed-off-by: Vinod Koul <vkoul@kernel.org>