net: use a more deterministic approach to get the active ethernet device

If the environment variable "ethact" is not set, the first device in the
uclass is returned. This depends on the probing order of the ethernet
devices. Moreover it is not not configurable at all.

Try to return the ethernet device with sequence id 0 first which then
can be configured by the aliases in a device tree. Fall back to the old
mechanism in case of an error.

Signed-off-by: Michael Walle <michael@walle.cc>
Reviewed-by: Ramon Fried <rfried.dev@gmail.com>