s390/qeth: translate SETVLAN/DELVLAN errors

Properly return any error encountered during VLAN processing to the
the caller.
Resulting change in behaviour: if SETVLAN fails while registering a
new VLAN ID, the stack no longer creates the corresponding vlan device.

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