net: dsa: mv88e6xxx: Free ATU/VTU irq only when there is chip irq

We only register the ATU and VTU irq when we have a chip level IRQ.
In the error path, we should only attempt to remove the ATU and VTU
irq if we also have a chip level IRQ.

Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>