8139too: Allow using the largest possible MTU

This driver allows MTU up to 1518 bytes which is not enought to run
batman-adv. Simply raise the maximum packet size up to the maximum
allowed by the transmit descriptor, 1792 bytes, giving a maximum MTU
of 1774 bytes.
Signed-off-by: Alban Bedel <albeu@free.fr>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/ethernet/realtek/8139too.c

@@ -183,7 +183,7 @@ static int debug = -1;
 #define NUM_TX_DESC	4
 
 /* max supported ethernet frame size -- must be at least (dev->mtu+14+4).*/
-#define MAX_ETH_FRAME_SIZE	1536
+#define MAX_ETH_FRAME_SIZE	1792
 
 /* max supported payload size */
 #define MAX_ETH_DATA_SIZE	(MAX_ETH_FRAME_SIZE - ETH_HLEN - ETH_FCS_LEN)