s390/qeth: move qdio's QAOB cache into qeth

qdio.ko no longer needs to care about how the QAOBs are allocated,
from its perspective they are merely another parameter to do_QDIO().

So for a start, shift the cache into the only qdio driver that uses
QAOBs (ie. qeth). Here there's further opportunity to optimize its
usage in the future - eg. make it per-{device, TX queue}, or only
compile it when the driver is built with CQ/QAOB support.

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