[S390] cpu hotplug: fix external interrupt subclass mask handling

When disabling a cpu all external interrupt subclass masks in control
register 0 get cleared. However instead of the service signal subclass
mask bit an unused bit got cleared.
Accidently (or luckily) the service subclass mask gets cleared with the
pfault_fini() call that happens just before the rest of the subclass
mask bits get cleared.

Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>