[media] v4l2-ctrl: fix error return of copy_to/from_user

copy_to/from_user returns the number of bytes not copied, it does not
return a 'normal' linux error code.

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