s390/qdio: use atomic_cmpxchg

qdio uses atomic_read to find an unused indicator and atomic_set to
flag it as used. This could lead to multiple users getting the same
indicator. Use atomic_cmpxchg instead.

Signed-off-by: Sebastian Ott <sebott@linux.vnet.ibm.com>
Acked-by: Ursula Braun <ubraun@linux.vnet.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>