net: dsa: mv88e6xxx: Return error from irq_find_mapping()

Fix a cut/paste error. When irq_find_mapping() returns an error for
the ATU or VTU interrupt, return that error, not the value of
chip->device_irq.

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