net: dsa: mv88e6xxx: Fix IRQ when loading module

Handle polled interrupts correctly when loading the module.

Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Fixes: 294d711e ("net: dsa: mv88e6xxx: Poll when no interrupt defined")
Signed-off-by: David S. Miller <davem@davemloft.net>