net/fec: prevent dobule restart of interface on FDX/HDX change

Upon detection of a FDX/HDX change the interface is restarted twice.

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>