[media] gspca: Fix underflow in vidioc_s_parm()

"n" is a user controlled integer.  The code here doesn't handle the case
where "n" is negative and this causes a static checker warning.

	drivers/media/usb/gspca/gspca.c:1571 vidioc_s_parm()
	warn: no lower bound on 'n'

parm.capture.readbuffers is unsigned, so make n unsigned too, fixing this.
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>

drivers/media/usb/gspca/gspca.c

@@ -1562,7 +1562,7 @@ static int vidioc_s_parm(struct file *filp, void *priv,
 			struct v4l2_streamparm *parm)
 {
 	struct gspca_dev *gspca_dev = video_drvdata(filp);
-	int n;
+	unsigned int n;
 
 	n = parm->parm.capture.readbuffers;
 	if (n == 0 || n >= GSPCA_MAX_FRAMES)