net: fec: Add Scatter/gather support

Add Scatter/gather support for FEC.
This feature allows to improve outbound throughput performance.

Tested on imx6dl sabresd board:
Running iperf tests shows a 55.4% improvement.

$ ethtool -K eth0 sg off
$ iperf -c 10.192.242.167 -t 3 &
[  3] local 10.192.242.108 port 52618 connected with 10.192.242.167 port 5001
[ ID] Interval       Transfer     Bandwidth
[  3]  0.0- 3.0 sec  99.5 MBytes   278 Mbits/sec

$ ethtool -K eth0 sg on
$ iperf -c 10.192.242.167 -t 3 &
[  3] local 10.192.242.108 port 52617 connected with 10.192.242.167 port 5001
[ ID] Interval       Transfer     Bandwidth
[  3]  0.0- 3.0 sec   154 MBytes   432 Mbits/sec

CC: Li Frank <B20596@freescale.com>
Signed-off-by: Fugang Duan <B38611@freescale.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/ethernet/freescale/fec.h

@@ -221,7 +221,7 @@ struct bufdesc_ex {
 #define BD_ENET_TX_RCMASK       ((ushort)0x003c)
 #define BD_ENET_TX_UN           ((ushort)0x0002)
 #define BD_ENET_TX_CSL          ((ushort)0x0001)
-#define BD_ENET_TX_STATS        ((ushort)0x03ff)        /* All status bits */
+#define BD_ENET_TX_STATS        ((ushort)0x0fff)        /* All status bits */
 
 /*enhanced buffer descriptor control/status used by Ethernet transmit*/
 #define BD_ENET_TX_INT          0x40000000