net/fec: don't request invalid IRQ

prevent calling request_irq() with a known invalid IRQ number and
preserve the return value of the platform_get_irq() function

Signed-off-by: Lothar Waßmann <LW@KARO-electronics.de>
Acked-by: Shawn Guo <shawn.guo@linaro.org>
Signed-off-by: David S. Miller <davem@davemloft.net>