net: dsa: mv88e6xxx: Fix unconditional irq freeing

Trying to remove an IRQ domain that was not created results in an
Opps. Add the necessary checks that the irqs were created before
freeing them.

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