s390/qeth: don't init refcount twice for mcast IPs

mcast IP objects are allocated within qeth_l3_add_mcast_rtnl(),
with .ref_counter already set to 1 via qeth_l3_init_ipaddr().

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