net: fec: only check queue 0 if RXF_0/TXF_0 interrupt is set

Before queue 0 was always checked if any queue caused an interrupt.
It is better to just mark queue 0 if queue 0 has caused an interrupt.

Signed-off-by: Troy Kisky <troy.kisky@boundarydevices.com>
Acked-by: Fugang Duan <Fugang.duan@nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>