s390/qeth: fix MAC address update sequence

When changing the MAC address on a L2 qeth device, current code first
unregisters the old address, then registers the new one.
If HW rejects the new address (or the IO fails), the device ends up with
no operable address at all.

Re-order the code flow so that the old address only gets dropped if the
new address was registered successfully. While at it, add logic to catch
some corner-cases.

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