net: dsa: mv88e6xxx: Fix irq free'ing

Call the common irq free function, rather than going recursive and
blowing away the stack, followed by the machine.

Fixes: 294d711e

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