s390/qeth: remove QETH_QDIO_BUF_HANDLED_DELAYED state

Reuse the QETH_QDIO_BUF_EMPTY state to indicate that a TX buffer has
been completed with a QAOB notification, and may be cleaned up by
qeth_cleanup_handled_pending().

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