net: mv643xx_eth: Implement software TSO

Now that the TSO helper API has been introduced, this commit makes use
of it to add support for software TSO in this driver.

This feature allows to improve outbound throughput performance significantly.
Running iperf tests shows a 30% improvement, tested on a Kirkwood Openblocks
A6 board.

$ ethtool -K eth0 tso off
$ iperf -c 192.168.0.45 -t 3
------------------------------------------------------------
Client connecting to 192.168.0.45, TCP port 5001
TCP window size: 43.8 KByte (default)
------------------------------------------------------------
[  3] local 192.168.0.159 port 46389 connected with 192.168.0.45 port 5001
[ ID] Interval       Transfer     Bandwidth
[  3]  0.0- 3.0 sec   217 MBytes   607 Mbits/sec

$ ethtool -K eth0 tso on
$ iperf -c 192.168.0.45 -t 3
------------------------------------------------------------
Client connecting to 192.168.0.45, TCP port 5001
TCP window size: 43.8 KByte (default)
------------------------------------------------------------
[  3] local 192.168.0.159 port 46390 connected with 192.168.0.45 port 5001
[ ID] Interval       Transfer     Bandwidth
[  3]  0.0- 3.0 sec   336 MBytes   938 Mbits/sec

This commit is just an example of the usage of the TSO API, it works fine
but needs some more work. In particular, the descriptor unmapping path must
avoid unmapping the TSO headers.

Signed-off-by: Ezequiel Garcia <ezequiel.garcia@free-electrons.com>
Signed-off-by: David S. Miller <davem@davemloft.net>