net: lantiq_xrx200: Hardcode the burst length value

All SoCs with this IP core support 8 burst length. Hauke
suggested to hardcode this value and simplify the driver.

Link: https://lkml.org/lkml/2021/9/14/1533
Signed-off-by: Aleksander Jan Bajkowski <olek2@wp.pl>
Signed-off-by: David S. Miller <davem@davemloft.net>