s390/qeth: consolidate L3 mcast registration code

Current code processes each (VLAN) device twice - once to inspect the
IPv4 mcast addresses, and then a second time to walk the IPv6 mcast
addresses. Unify all this into a single helper, thus removing some
checks and a duplicated VLAN lookup.

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