s390x/ipl: avoid sign extension

Make s390_update_iplstate() return uint32_t to avoid sign extensions
for cssids > 127. While this doesn't matter in practice yet (as
nobody supports MCSS-E and thus won't see the real cssid), play safe.
Reported-by: NPaolo Bonzini <pbonzini@redhat.com>
Reviewed-by: NJason J. Herne <jjherne@linux.vnet.ibm.com>
Signed-off-by: NCornelia Huck <cornelia.huck@de.ibm.com>

hw/s390x/ipl.c

@@ -218,7 +218,7 @@ static Property s390_ipl_properties[] = {
  * - -1 if no valid boot device was found
  * - ccw id of the boot device otherwise
  */
-static uint64_t s390_update_iplstate(CPUS390XState *env, S390IPLState *ipl)
+static uint32_t s390_update_iplstate(CPUS390XState *env, S390IPLState *ipl)
 {
     DeviceState *dev_st;