gianfar: Enable eTSEC-20 erratum w/a for P2020 Rev1

Enable workaround for P2020/P2010 erratum eTSEC 20,
"Excess delays when transmitting TOE=1 large frames".
The impact is that frames lager than 2500-bytes for which
TOE (i.e. TCP/IP hw accelerations like Tx csum) is enabled
may see excess delay before start of transmission.
This erratum was fixed in Rev 2.0.

Signed-off-by: Claudiu Manoil <claudiu.manoil@freescale.com>
Signed-off-by: David S. Miller <davem@davemloft.net>