soundwire: bus: export broadcast read/write capability for tests

soundwire: bus: export broadcast read/write capability for tests

Provide prototype and export symbol to enable tests. The bus lock is
handled externally to avoid conflicts e.g. between kernel-generated
traffic and test traffic.

Signed-off-by: Pierre-Louis Bossart <pierre-louis.bossart@linux.intel.com>
Signed-off-by: Bard Liao <yung-chuan.liao@linux.intel.com>
Reviewed-by: Kai Vehmanen <kai.vehmanen@linux.intel.com>
Reviewed-by: Guennadi Liakhovetski <guennadi.liakhovetski@linux.intel.com>
Link: https://lore.kernel.org/r/20200908134521.6781-7-yung-chuan.liao@linux.intel.com
Signed-off-by: Vinod Koul <vkoul@kernel.org>