net: Initialize as many ethernet devices as possible

On devices that have their first network interface provided by a FPGA,
the initialization of further interfaces will fail if the FPGA is not
yet programmed. This leads to problems during factory setup when the
data is supposed to be loaded over secondary netowork interfaces.

To avoid this, use the uclass_{first,next}_device_check functions to
initialize as many ethernet devices as possible.

Signed-off-by: Mario Six <mario.six@gdsys.cc>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>