net: axienet: Support shared interrupts

Specify IRQF_SHARED to support shared interrupts. If the interrupt
handler is called and the device is not indicating an interrupt,
just return IRQ_NONE rather than spewing error messages.

Signed-off-by: Robert Hancock <hancock@sedsystems.ca>
Signed-off-by: David S. Miller <davem@davemloft.net>