Even if the setting of the MAC address does work correctly, the function
return an error.

In the former barebox version, this didn't seem to be a problem. In
v2015.11.0 version, the "ifup eth0" command fails because of this, and
as a consequence the network interface doesn't work.

The fix is straightforward.

Signed-off-by: Robert Jarzmik <robert.jarzmik@free.fr>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>