Fix W=1 warnings, add missing field descriptions.

Signed-off-by: Pierre-Louis Bossart <pierre-louis.bossart@linux.intel.com>
Link: https://lore.kernel.org/r/20200702193604.169059-20-pierre-louis.bossart@linux.intel.com


Signed-off-by: Takashi Iwai <tiwai@suse.de>