media: vicodec: fix out-of-range values when decoding

While decoding you need to make sure you do not get values < 0
or > 255. Note that since this code will also be used in userspace
utilities the clamp macro isn't used since that is kernel-only.

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