s390/qeth: advertise IFF_UNICAST_FLT

qeth implements HW-based Unicast Filtering (via SETVMAC) on L2 devices.
Tell the stack, so it knows that receiving traffic for secondary
addresses doesn't require full-blown promiscuous mode.

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