soundwire: intel: move shutdown() callback and don't export symbol

All DAI callbacks are in intel.c except for shutdown. Move and remove
export symbol

Signed-off-by: Pierre-Louis Bossart <pierre-louis.bossart@linux.intel.com>
Link: https://lore.kernel.org/r/20190806005522.22642-18-pierre-louis.bossart@linux.intel.com


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