s390/qeth: consolidate completion of pending TX buffers

With commit 396c1004 ("s390/qdio: let driver manage the QAOB")
a pending TX buffer now has access to its associated QAOB during
TX completion processing. We can thus reduce the amount of work & state
propagation that needs to be done by qeth_qdio_handle_aob().

Move all this logic into the respective TX completion paths. Doing so
even allows us to determine more precise TX_NOTIFY_* values via
qeth_compute_cq_notification(aob->aorc, ...).

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