drivers: eth: native_posix: Allow user to manipulate zeth status

User can take the zeth interface down by issuing "net iface down <idx>"
shell command. It is possible to take the interface up by typing
"net iface up <idx>" in shell. These commands are important for
native_posix as there is no physical cable that can be connected
or disconnected.

Signed-off-by: Jukka Rissanen <jukka.rissanen@linux.intel.com>