[S390] qdio: dont cast function pointers and use them to call functions.

According to C99 6.3.2.3 it's undefined what happens if a converted
pointer is used to call a function whose type is not compatible with
the pointed-to type.
That's what the qdio code is doing, so fix it.

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