ar71xx/ath79: ag71xx: dont fetch the same var again

tx_size was just declared above and set to BIT(tx->order)
Use the declaration instead, which could avoid a pointer deref

Signed-off-by: Koen Vandeputte <koen.vandeputte@ncentric.com>
(cherry picked from commit 6852eab58236e3991f5d73f211018dfef01f2a64)

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

@@ -298,7 +298,7 @@ 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;
 

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

@@ -284,7 +284,7 @@ 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;