Staging: et131x: unify return value of .ndo_set_mac_address if address is invalid

Unify return value of .ndo_set_mac_address if the given address
isn't valid. Return -EADDRNOTAVAIL as eth_mac_addr() already does
if is_valid_ether_addr() fails.

Signed-off-by: Danny Kukawka <danny.kukawka@bisect.de>
Signed-off-by: David S. Miller <davem@davemloft.net>