ar71xx/ath79: ag71xx: get ring_mask consistent

All other instances of this identical declaration fetch the
value directly from the ring_order.

Also do it here.

Signed-off-by: Koen Vandeputte <koen.vandeputte@ncentric.com>

target/linux/ar71xx/files/drivers/net/ethernet/atheros/ag71xx/ag71xx_main.c

@@ -127,7 +127,7 @@ static void ag71xx_ring_tx_init(struct ag71xx *ag)
 {
 	struct ag71xx_ring *ring = &ag->tx_ring;
 	int ring_size = BIT(ring->order);
-	int ring_mask = ring_size - 1;
+	int ring_mask = BIT(ring->order) - 1;
 	int i;
 
 	for (i = 0; i < ring_size; i++) {

target/linux/ath79/files/drivers/net/ethernet/atheros/ag71xx/ag71xx_main.c

@@ -130,7 +130,7 @@ static void ag71xx_ring_tx_init(struct ag71xx *ag)
 {
 	struct ag71xx_ring *ring = &ag->tx_ring;
 	int ring_size = BIT(ring->order);
-	int ring_mask = ring_size - 1;
+	int ring_mask = BIT(ring->order) - 1;
 	int i;
 
 	for (i = 0; i < ring_size; i++) {