[media] v4l2-mediabus.h: use two __u16 instead of two __u32

The ycbcr_enc and quantization fields do not need a __u32. Switch to
two __u16 types, thus preserving alignment and avoiding holes in the
struct. This makes one more __u32 available for future expansion.

Suggested by Sakari Ailus.

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>