net: ftgmac100: Disable hardware checksum on AST2600

The AST2600 when using the i210 NIC over NC-SI has been observed to
produce incorrect checksum results with specific MTU values. This was
first observed when sending data across a long distance set of networks.

On a local network, the following test was performed using a 1MB file of
random data.

On the receiver run this script:

 #!/bin/bash
 while [ 1 ]; do
        # Zero the stats
        nstat -r  > /dev/null
        nc -l 9899 > test-file
        # Check for checksum errors
        TcpInCsumErrors=$(nstat | grep TcpInCsumErrors)
        if [ -z "$TcpInCsumErrors" ]; then
                echo No TcpInCsumErrors
        else
                echo TcpInCsumErrors = $TcpInCsumErrors
        fi
 done

On an AST2600 system:

 # nc <IP of  receiver host> 9899 < test-file

The test was repeated with various MTU values:

 # ip link set mtu 1410 dev eth0

The observed results:

 1500 - good
 1434 - bad
 1400 - good
 1410 - bad
 1420 - good

The test was repeated after disabling tx checksumming:

 # ethtool -K eth0 tx-checksumming off

And all MTU values tested resulted in transfers without error.

An issue with the driver cannot be ruled out, however there has been no
bug discovered so far.

David has done the work to take the original bug report of slow data
transfer between long distance connections and triaged it down to this
test case.

The vendor suspects this this is a hardware issue when using NC-SI. The
fixes line refers to the patch that introduced AST2600 support.

Reported-by: David Wilder <wilder@us.ibm.com>
Reviewed-by: Dylan Hung <dylan_hung@aspeedtech.com>
Signed-off-by: Joel Stanley <joel@jms.id.au>
Signed-off-by: David S. Miller <davem@davemloft.net>