[media] V4L: fix the Z16 format definition

A copy paste error created that format with the same one-line
description as Y8I and Y12I, whereas Z16 is quite different from them
both.

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