korina: count RX DMA OVR as rx_fifo_error

This way, RX DMA overruns (actually being caused by overrun of the
512byte input FIFO) show up in ifconfig output. The rx_fifo_errors
counter is unused otherwise.

Signed-off-by: Phil Sutter <phil@nwl.cc>
Signed-off-by: David S. Miller <davem@davemloft.net>