KVM: s390: Fix size of monitor-class number field

The monitor-class number field is only 16 bits, so we have to use
a u16 pointer to access it.

Signed-off-by: Thomas Huth <thuth@linux.vnet.ibm.com>
Reviewed-by: David Hildenbrand <dahi@linux.vnet.ibm.com>
CC: stable@vger.kernel.org # v3.16+
Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>