s390/qeth: use IQD Multi-Write

For IQD devices with Multi-Write support, we can defer the queue-flush
further and transmit multiple IO buffers with a single TX doorbell.
The same-target restriction still applies.

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