[S390] qdio: Sanitize do_QDIO sanity checks

Remove unneeded sanity checks from do_QDIO since this is the hot path.
Change the type of bufnr and count to unsigned int so the check for the
maximum value works.

Reported-by: Roel Kluin <roel.kluin@gmail.com>
Signed-off-by: Jan Glauber <jang@linux.vnet.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>