s390/smp: fix smp_stop_cpu() for !CONFIG_SMP

s390/smp: fix smp_stop_cpu() for !CONFIG_SMP

smp_stop_cpu() should stop the current cpu even for !CONFIG_SMP.
Otherwise machine_halt() will return and and the machine generates a
panic instread of simply stopping the current cpu:

Kernel panic - not syncing: Attempted to kill init! exitcode=0x00000000

CPU: 0 PID: 1 Comm: systemd-shutdow Not tainted 3.14.0-01527-g2b6ef16a6bc5 #10
[...]
Call Trace:
([<0000000000110db0>] show_trace+0xf8/0x158)
 [<0000000000110e7a>] show_stack+0x6a/0xe8
 [<000000000074dba8>] panic+0xe4/0x268
 [<0000000000140570>] do_exit+0xa88/0xb2c
 [<000000000016e12c>] SyS_reboot+0x1f0/0x234
 [<000000000075da70>] sysc_nr_ok+0x22/0x28
 [<000000007d5a09b4>] 0x7d5a09b4

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