[media] videodev2: Set vb2_rect's width and height as unsigned

As discussed on the media summit 2013, there is no reason for the width
and height to be signed.

Therefore this patch is an attempt to convert those fields from __s32 to
__u32.

Signed-off-by: Ricardo Ribalda Delgado <ricardo.ribalda@gmail.com>
Acked-by: Sakari Ailus <sakari.ailus@iki.fi> (documentation and smiapp)
Acked-by: Lad, Prabhakar <prabhakar.csengg@gmail.com>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>