net: lantiq: configure the burst length in ethernet drivers

Configure the burst length in Ethernet drivers. This improves
Ethernet performance by 58%. According to the vendor BSP,
8W burst length is supported by ar9 and newer SoCs.

The NAT benchmark results on xRX200 (Down/Up):
* 2W: 330 Mb/s
* 4W: 432 Mb/s    372 Mb/s
* 8W: 520 Mb/s    389 Mb/s

Tested on xRX200 and xRX330.

Signed-off-by: Aleksander Jan Bajkowski <olek2@wp.pl>
Signed-off-by: David S. Miller <davem@davemloft.net>