s390/qeth: qdio queue helpers

Get rid of duplicated code by introducing some helpers to allocate
and free qdio queues used by qeth for its input and completion queue.

No functional change.

Reviewed-by: Ursula Braun <ursula.braun@de.ibm.com>
Signed-off-by: Sebastian Ott <sebott@linux.vnet.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>