device_probe() may fail in which case the seq_id will be -1. Don't
display these devices during startup. While this is only a cosmetic
change, the return value of eth_initialize() will also change to the
actual number of available devices. The return value is only used in
spl_net to decide whether there are any devices to boot from. So
returning only available devices is also more correct in that case.

Signed-off-by: Michael Walle <michael@walle.cc>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>