qeth: meaningful return code for set_mac_address

Setting an invalid mac-address for a qeth layer2 device returns
with a strange error code:
# ip link set hsi1 address 00:00:00:00:00:01
   RTNETLINK answers: Unknown error 18446744073709486085
Problem is caused by wrong usage of the return_code field within
structure qeth_ipa_cmd.
With this patch the ip command above returns
   SIOCSIFHWADDR: Invalid argument

Signed-off-by: Ursula Braun <ursula.braun@de.ibm.com>
Signed-off-by: Frank Blaschka <frank.blaschka@de.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>