net: ag71xx: Slighly simplify code in 'ag71xx_rings_init()'

A few lines above, we have:
   tx_size = BIT(tx->order);

So use 'tx_size' directly to be consistent with the way 'rx->descs_cpu' and
'rx->descs_dma' are computed below.
Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/ethernet/atheros/ag71xx.c

@@ -1148,7 +1148,7 @@ static int ag71xx_rings_init(struct ag71xx *ag)
 		return -ENOMEM;
 	}
 
-	rx->buf = &tx->buf[BIT(tx->order)];
+	rx->buf = &tx->buf[tx_size];
 	rx->descs_cpu = ((void *)tx->descs_cpu) + tx_size * AG71XX_DESC_SIZE;
 	rx->descs_dma = tx->descs_dma + tx_size * AG71XX_DESC_SIZE;