s390/qeth: consolidate open/stop netdev ops

The L2 and L3 code for these ops is almost identical, we only need to
provide a custom ndo_validate_addr() for L2 that checks whether
programming the MAC address succeeded.

Signed-off-by: Julian Wiedmann <jwi@linux.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>