net: ethernet: mediatek: remove redundant free_irq for devm_request_irq allocated irq

these irqs are not used for shared irq and disabled during ethernet stops.
irq requested by devm_request_irq is safe to be freed automatically on
driver detach.

Signed-off-by: Sean Wang <sean.wang@mediatek.com>
Acked-by: John Crispin <john@phrozen.org>
Signed-off-by: David S. Miller <davem@davemloft.net>