[S390] qdio: fix EQBS handling on CCQ96

QDIO returned from EQBS instruction in any case after return code
CCQ=96 was issued regardless whether buffer states for at least one
buffer were extracted or not.
This caused FCP devices to hang when running under z/VM and having
QIOASSASIST=ON and having high I/O rates.
In order to fix this qdio return code processing of EQBS instruction
after CCQ=96 is changed that buffers are returned and if no buffers
where extracted the instruction is repeated at once.

Signed-off-by: Klaus D. Wacker <kdwacker@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>
Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>