[S390] sclp: undo quiesce handler override on resume

In a system where the ctrl-alt-del init action initiated by signal
quiesce suspends the machine the quiesce handler override for
_machine_restart, _machine_halt and _machine_power_off needs to be
undone, otherwise the override is still present in the resumed
system. The next shutdown would then load the quiesce state psw
instead of performing the correct shutdown action.

Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>