qeth: avoid duplicate deletion of multicast addresses
if qeth_set_multicast_list() is performed on 2 CPUs in parallel,
card->ip_list may end corrupted.
Solution: In function __qeth_delete_all_mc()
remove card->ip_list entry before invoking
qeth_deregister_addr_entry(). Thus a 2nd invocation of
qeth_set_multicast_list() cannot try to remove the
same entry twice.