media: Use designated initializers for all subdev pad ops

Structures passed to subdev pad operations are all zero-initialized when
declaring variables. In most cases, this is done with designated
initializers to initialize some of the fields to specific values, but in
a minority of cases the structures are zero-initialized by assigning
them to '{ 0 }' or '{ }'.

Improve coding style consistency by using designated initializers where
possible, always initializing the 'which' field.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Acked-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Reviewed-by: Tomi Valkeinen <tomi.valkeinen@ideasonboard.com>
Reviewed-by: Kieran Bingham <kieran.bingham+renesas@ideasonboard.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>