s390/qeth: simplify QoS code

qeth_get_priority_queue() is no longer used for IQD devices, remove the
special-casing of their mcast queue.

This effectively reverts
commit 70deb016

 ("qeth: omit outbound queue 3 for unicast packets in Priority Queuing on HiperSockets").

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