[media] xc4000: shut up a bogus smatch message

smatch complains about:
	drivers/media/tuners/xc4000.c:1511 xc4000_get_signal() warn: '~value << 3' 524280 can't fit into 65535 'value'

Remove the bogus complain.
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>

drivers/media/tuners/xc4000.c

@@ -1508,7 +1508,7 @@ static int xc4000_get_signal(struct dvb_frontend *fe, u16 *strength)
 	if (value >= 0x2000) {
 		value = 0;
 	} else {
-		value = ~value << 3;
+		value = (~value << 3) & 0xffff;
 	}
 
 	goto ret;