net: moxa: Fix a potential double 'free_irq()'

Should an irq requested with 'devm_request_irq' be released explicitly,
it should be done by 'devm_free_irq()', not 'free_irq()'.

Fixes: 6c821bd9 ("net: Add MOXA ART SoCs ethernet driver")
Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Signed-off-by: David S. Miller <davem@davemloft.net>