soundwire: return earlier if no slave is attached

soundwire: return earlier if no slave is attached

If there is no slave attached to soundwire bus, we
can return earlier from sdw_bus_prep_clk_stop() and
sdw_bus_exit_clk_stop(), this saves a redundant value
check.

Signed-off-by: Chao Song <chao.song@linux.intel.com>
Reviewed-by: Rander Wang <rander.wang@intel.com>
Reviewed-by: Guennadi Liakhovetski <guennadi.liakhovetski@linux.intel.com>
Reviewed-by: Pierre-Louis Bossart <pierre-louis.bossart@linux.intel.com>
Signed-off-by: Bard Liao <yung-chuan.liao@linux.intel.com>
Link: https://lore.kernel.org/r/20210126085439.4349-1-yung-chuan.liao@linux.intel.com
Signed-off-by: Vinod Koul <vkoul@kernel.org>