s390/qdio: reset old sbal_state flags

When allocating a new AOB fails, handle_outbound() is still capable of
transmitting the selected buffer (just without async completion).

But if a previous transfer on this queue slot used async completion, its
sbal_state flags field is still set to QDIO_OUTBUF_STATE_FLAG_PENDING.
So when the upper layer driver sees this stale flag, it expects an async
completion that never happens.

Fix this by unconditionally clearing the flags field.

Fixes: 104ea556 ("qdio: support asynchronous delivery of storage blocks")
Cc: <stable@vger.kernel.org> #v3.2+
Signed-off-by: Julian Wiedmann <jwi@linux.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>