soundwire: bus: add bpt_stream pointer

Add a convenience pointer to the 'sdw_bus' structure. BPT is a
dedicated stream which will typically not be handled by DAIs or
dailinks. Since there's only one BPT stream per link, storing the
pointer at the link level seems rather natural.

Signed-off-by: Pierre-Louis Bossart <pierre-louis.bossart@linux.dev>
Signed-off-by: Bard Liao <yung-chuan.liao@linux.intel.com>
Reviewed-by: Péter Ujfalusi <peter.ujfalusi@linux.intel.com>
Reviewed-by: Liam Girdwood <liam.r.girdwood@intel.com>
Reviewed-by: Ranjani Sridharan <ranjani.sridharan@linux.intel.com>
Tested-by:  <shumingf@realtek.com>
Link: https://lore.kernel.org/r/20250227140615.8147-9-yung-chuan.liao@linux.intel.com


Signed-off-by: Vinod Koul <vkoul@kernel.org>