cvideo: Fix control value clamping for unsigned integer controls

V4L2 integer controls are stored in signed 32-bit values. However, UVC
controls can be either signed or unsigned. Take the UVC control
signedness into account when clamping the control value to the min-max
range.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>
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