net: eth_write_hwaddr: Return error for invalid MACs

If dev->enetaddr was supposed to be set with dev->write_hwaddr() but the MAC
address was not valid, return an error.

Signed-off-by: Benoît Thébaudeau <benoit.thebaudeau@advansee.com>
Cc: Joe Hershberger <joe.hershberger@gmail.com>
Acked-by: Mike Frysinger <vapier@gentoo.org>