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: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Jason J. Herne <jjherne@linux.vnet.ibm.com>
Signed-off-by: Cornelia Huck <cornelia.huck@de.ibm.com>