net: axienet: reduce default RX interrupt threshold to 1

Now that NAPI has been implemented, the hardware interrupt mitigation
mechanism is not needed to avoid excessive interrupt load in most cases.
Reduce the default RX interrupt threshold to 1 to reduce introduced
latency. This can be increased with ethtool if desired if some applications
still want to reduce interrupts.

Signed-off-by: Robert Hancock <robert.hancock@calian.com>
Signed-off-by: David S. Miller <davem@davemloft.net>