Previously, when invalid address was passed to ndo_set_mac_address,
random mac was generated and set. Fix this by returning -EADDRNOTAVAIL
in this situation.

Also polish the code around a bit.
Signed-off-by: Jiri Pirko <jiri@resnulli.us>
Signed-off-by: David S. Miller <davem@davemloft.net>