net: ethernet: xilinx: Do not use NO_IRQ in axienet

This driver is used on Microblaze and will be used
on Arm Zynq.
Microblaze doesn't define NO_IRQ and no IRQ is 0.
Arm still uses NO_IRQ as -1 and there is no option
to connect IRQ to irq 0.

That's why <= 0 is only one option how to find out
undefined IRQ.

Signed-off-by: Michal Simek <michal.simek@xilinx.com>
Signed-off-by: David S. Miller <davem@davemloft.net>