cxgb3: FIx VLAN over Jumbo frames

The mac is expected to auto-inflate the Maximum Frame size for VLAN
tagged frames. It however does not work with jumbo frames.
Work around the bug adding 4 to the Maximum Frame for MTUs
greater than 1536.
Signed-off-by: NDivy Le Ray <divy@chelsio.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

drivers/net/cxgb3/xgmac.c

@@ -353,6 +353,9 @@ int t3_mac_set_mtu(struct cmac *mac, unsigned int mtu)
 	 * packet size register includes header, but not FCS.
 	 */
 	mtu += 14;
+	if (mtu > 1536)
+		mtu += 4;
+
 	if (mtu > MAX_FRAME_SIZE - 4)
 		return -EINVAL;
 	t3_write_reg(adap, A_XGM_RX_MAX_PKT_SIZE + mac->offset, mtu);