au1000_eth: Duplicate test of RX_OVERLEN bit in update_rx_stats()

in update_rx_stats() the RX_OVERLEN bit is set twice, replace it by RX_RUNT.
in au1000_rx() the RX_MISSED_FRAME bit was tested a few lines earlier already

Signed-off-by: Roel Kluin <roel.kluin@gmail.com>
Acked-by: Manuel Lauss <manuel.lauss@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>