[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>

drivers/s390/cio/qdio_setup.c

@@ -22,12 +22,9 @@
 static struct kmem_cache *qdio_q_cache;
 static struct kmem_cache *qdio_aob_cache;
 
-struct qaob *qdio_allocate_aob()
+struct qaob *qdio_allocate_aob(void)
 {
-	struct qaob *aob;
-
-	aob = kmem_cache_zalloc(qdio_aob_cache, GFP_ATOMIC);
-	return aob;
+	return kmem_cache_zalloc(qdio_aob_cache, GFP_ATOMIC);
 }
 EXPORT_SYMBOL_GPL(qdio_allocate_aob);