[media] v4l: Fix colorspace conversion error in sample code

The sample code erroneously scales the y1, pb and pr variables from the
[0.0 .. 1.0] and [-0.5 .. 0.5] ranges to [0 .. 255] and [-128 .. 127].
Fix it.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Acked-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>