s390/qeth: reject multicast rxip addresses

There exist different commands to add unicast and multicast addresses on
the OSA card. rxip addresses are always set as unicast addresses and
thus just unicast addresses should be allowed.

Adding a multicast address now fails and a grace message is generated.

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