Handle the RX FIFO overflow condition with the rxeof handler instead

Handle the RX FIFO overflow condition with the rxeof handler instead
of the rxeoc handler for now. The rxeoc handler will reset the link,
and the NatSemi chip's RX FIFO will overflow on a 32-bit bus once you
start hitting it with 500Mbps or more of traffic.

Also increase the size of the RX ring to 128 descriptors (was 64).