s390/qdio: make qdio_handle_aobs() more robust

When processing a PENDING buffer with no attached aob, the current code
would get stuck on this buffer (as the 'continue' causes us to not
advance the buffer index) and process it repeatedly until the loop
terminates eventually.

Luckily this should never happen - the HW must not use the PENDING state
when no aob was provided. But we can still make this code path less
fragile and protect against buggy devices.

Signed-off-by: Julian Wiedmann <jwi@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>