[S390] qdio: fix qdio_allocate_aob() declaration

drivers/s390/cio/qdio_setup.c:24:32:
  warning: non-ANSI function declaration of function 'qdio_allocate_aob'

While at it also simplify the function.

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