soundwire: stream: move list addition to sdw_slave_alloc_rt()

soundwire: stream: move list addition to sdw_slave_alloc_rt()

Simplify sdw_stream_add_slave() by moving the linked list management
inside of the sdw_slave_alloc_rt_free() helper, this also makes the
alloc/free helpers more symmetrical.

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/20220126011715.28204-16-yung-chuan.liao@linux.intel.com
Signed-off-by: Vinod Koul <vkoul@kernel.org>