soundwire: stream: introduce sdw_slave_rt_find() helper

soundwire: stream: introduce sdw_slave_rt_find() helper

Before we split the alloc and config steps, we need a helper to find
the Slave runtime for a stream. The helper is based on the search loop
in sdw_slave_rt_free(), which can now be simplified.

Signed-off-by: Pierre-Louis Bossart <pierre-louis.bossart@linux.intel.com>
Reviewed-by: Rander Wang <rander.wang@intel.com>
Signed-off-by: Bard Liao <yung-chuan.liao@linux.intel.com>
Link: https://lore.kernel.org/r/20220126011715.28204-18-yung-chuan.liao@linux.intel.com
Signed-off-by: Vinod Koul <vkoul@kernel.org>