netdev: Change netdev_get_etheraddr() to return an error code.

To make the netdev code more portable, it needs to support returning error
codes from functions that don't have them.  This commit changes
netdev_get_etheraddr() to return an error code and updates all of its
callers.

(Currently netdev_get_etheraddr() won't ever return an error, but other
future implementations might.)