media: uvcvideo: Introduce uvc_mapping_v4l2_size

Centralize the calculation for the v4l2_size of a mapping.

Reviewed-by: Yunke Cao <yunkec@google.com>
Tested-by: Yunke Cao <yunkec@google.com>
Reviewed-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Ricardo Ribalda <ribalda@chromium.org>
Link: https://lore.kernel.org/r/20250203-uvc-roi-v17-14-5900a9fed613@chromium.org


Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Hans Verkuil <hverkuil@xs4all.nl>