soundwire: intel: prevent possible dereference in hw_params

soundwire: intel: prevent possible dereference in hw_params

This should not happen in production systems but we should test for
all callback arguments before invoking the config_stream callback.

Update the prototype to clarify that the first argument is mandatory.

Also use local variable instead of multiple dereferences to improve
readability.

Signed-off-by: Pierre-Louis Bossart <pierre-louis.bossart@linux.intel.com>
Link: https://lore.kernel.org/r/20190806005522.22642-2-pierre-louis.bossart@linux.intel.com
Signed-off-by: Vinod Koul <vkoul@kernel.org>