soundwire: cadence: clear FIFO to avoid pop noise issue on playback start

Driver should clear FIFO in PDI, or the previously stored sample data
in FIFO will generate pop noise when stream is started. The soft reset
bit will clear all the FIFO to zero and is self-cleared after that.

Signed-off-by: randerwang <rander.wang@linux.intel.com>
Signed-off-by: Pierre-Louis Bossart <pierre-louis.bossart@linux.intel.com>
Link: https://lore.kernel.org/r/20200317163329.25501-18-pierre-louis.bossart@linux.intel.com


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