s390/qdio: consolidate QIB code

Move all QIB-related code into qdio_setup_qib(), and slightly re-order
it according to the order of the struct's fields. This makes it easier
to understand what the QIB actually looks like before we send it to HW.

Also get rid of the qebsm_possible() helper - as 31-bit support is long
gone, the comment doesn't make any sense. And while removing some stale
QIB-related comment, also move the clearing of the QDR into its proper
place.

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