media: docs: media: v4l: uapi: Fix field type for SUBDEV_ENUM_FRAME_SIZE

The documentation for the VIDIOC_SUBDEV_ENUM_FRAME_SIZE ioctl
incorrectly refers to struct v4l2_subdev_mbus_code_enum as the
parameter. Fix this.

[Sakari Ailus: Rebased on top of Dorota's documentation patch]

Signed-off-by: Paul Elder <paul.elder@ideasonboard.com>
Acked-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>