net/fec: fix fec1 check in fec_enet_mii_init()

In function fec_enet_mii_init(), it uses non-zero pdev->id as part
of the condition to check the second fec instance (fec1).  This works
before the driver supports device tree probe.  But in case of device
tree probe, pdev->id is -1 which is also non-zero, so the logic becomes
broken when device tree probe gets supported.

The patch change the logic to check "pdev->id > 0" as the part of the
condition for identifying fec1.

Signed-off-by: Shawn Guo <shawn.guo@linaro.org>
Signed-off-by: David S. Miller <davem@davemloft.net>