s390/qeth: add TX multiqueue support for OSA devices

This adds trivial support for multiple TX queues on OSA-style devices
(both real HW and z/VM NICs). For now we expose the driver's existing
QoS mechanism via .ndo_select_queue, and adjust the number of available
TX queues when qeth_update_from_chp_desc() detects that the
HW configuration has changed.

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