net: axienet: Service completion interrupts ASAP

The packet completion interrupts for TX and RX should be serviced before
the packets are consumed. This ensures against the degenerate case when a
new completion interrupt is raised after the handler has exited but before
the interrupts are cleared. In this case its possible for the ISR to clear
an unhandled interrupt (leading to potential deadlock).

Signed-off-by: Peter Crosthwaite <peter.crosthwaite@xilinx.com>
Tested-by: Jason Wu <huanyu@xilinx.com>
Acked-by: Michal Simek <michal.simek@xilinx.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>
Signed-off-by: David S. Miller <davem@davemloft.net>