net: korina: don't use overflow and underflow interrupts

When such interrupts occur there is not much we can do.
Dropping the whole ring doesn't help and only produces high packet loss.
If we just ignore the interrupt the mac will drop one or few packets instead of the whole ring.
Also this will lower the irq handling load and increase performance.

Signed-off-by: Roman Yeryomin <roman@advem.lv>
Signed-off-by: David S. Miller <davem@davemloft.net>