V4L/DVB (13868): gspca - sn9c20x: Fix test of unsigned.

Signed-off-by: Roel Kluin <roel.kluin@gmail.com>
Signed-off-by: Jean-Francois Moine <moinejf@free.fr>
CC: stable@kernel.org
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>

drivers/media/video/gspca/sn9c20x.c

@@ -2319,7 +2319,7 @@ static void do_autogain(struct gspca_dev *gspca_dev, u16 avg_lum)
 		}
 	}
 	if (avg_lum > MAX_AVG_LUM) {
-		if (sd->gain - 1 >= 0) {
+		if (sd->gain >= 1) {
 			sd->gain--;
 			set_gain(gspca_dev);
 		}