media: v4l2-tpg: use __u16 instead of int for struct tpg_rbg_color16

Despite the struct says "color16", it was actually using 32 bits
for each color. Fix it.

Suggested-by: Hans Verkuil <hverkuil@xs4all.nl>
Acked-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>