net/eth: fix a bug in on_ethaddr()

The loop should check all ethenet devices, not only the first device,
to set each specified ethaddr, or it'll cause failure when we use other
devices.

Signed-off-by: Gong Qianyu <Qianyu.Gong@freescale.com>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>