soundwire: intel: add prepare support in sdw dai driver

The existing code does not expose a prepare operation, which is very
much needed to deal with underflow and resume operations.

Signed-off-by: Pierre-Louis Bossart <pierre-louis.bossart@linux.intel.com>
Signed-off-by: Rander Wang <rander.wang@linux.intel.com>
Acked-by: Sanyog Kale <sanyog.r.kale@intel.com>
Link: https://lore.kernel.org/r/20200215014740.27580-3-pierre-louis.bossart@linux.intel.com


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