s390/qeth: handle limited IPv4 broadcast in L3 TX path

When selecting the cast type of a neighbourless IPv4 skb (eg. on a raw
socket), qeth_l3 falls back to the packet's destination IP address.
For this case we should classify traffic sent to 255.255.255.255 as
broadcast.
This fixes DHCP requests, which were misclassified as unicast
(and for IQD interfaces thus ended up on the wrong HW queue).

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