netdev: Fix carrier status for down interfaces.

netdev: Fix carrier status for down interfaces.

Currently netdev_get_carrier() returns both a carrier status and
an error code.  However, usage of the error code was inconsistent:
most callers either ignored it or didn't perform their task if an
error occured, which prevented bond rebalancing.  This makes the
handling consistent by translating an error into a down status in
the netdev library.

Bug #3959