net: lantiq_xrx200: fix statistics of received bytes

Received frames have FCS truncated. There is no need
to subtract FCS length from the statistics.

Fixes: fe1a5642 ("net: lantiq: Add Lantiq / Intel VRX200 Ethernet driver")
Signed-off-by: Aleksander Jan Bajkowski <olek2@wp.pl>
Signed-off-by: David S. Miller <davem@davemloft.net>