media: v4l2-subdev: clear reserved fields

Clear the reserved fields for these ioctls according to the specification:

VIDIOC_SUBDEV_ENUM_FRAME_INTERVAL
VIDIOC_SUBDEV_ENUM_FRAME_SIZE
VIDIOC_SUBDEV_ENUM_MBUS_CODE
VIDIOC_SUBDEV_G_CROP, VIDIOC_SUBDEV_S_CROP
VIDIOC_SUBDEV_G_FMT, VIDIOC_SUBDEV_S_FMT
VIDIOC_SUBDEV_G_FRAME_INTERVAL, VIDIOC_SUBDEV_S_FRAME_INTERVAL
VIDIOC_SUBDEV_G_SELECTION, VIDIOC_SUBDEV_S_SELECTION

Found with v4l2-compliance.

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Acked-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>