s390/qeth: remove prio-queueing support for z/VM NICs

z/VM NICs don't offer HW QoS for TX rings. So just use netdev_pick_tx()
to distribute the connections equally over all enabled TX queues.

We start with just 1 enabled TX queue (this matches the typical
configuration without prio-queueing). A follow-on patch will allow users
to enable additional TX queues.

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