MIPS: No need to check CPU 0 in cps_cpu_disable()

After commit 9cce844a ("MIPS: CPU#0 is not hotpluggable"),
c->hotpluggable is 0 for CPU 0 and it will not generate a control
file in sysfs for this CPU:

[root@linux loongson]# cat /sys/devices/system/cpu/cpu0/online
cat: /sys/devices/system/cpu/cpu0/online: No such file or directory
[root@linux loongson]# echo 0 > /sys/devices/system/cpu/cpu0/online
bash: /sys/devices/system/cpu/cpu0/online: Permission denied

So no need to check CPU 0 in cps_cpu_disable(), just remove it.
Reported-by: liwei (GF) <liwei391@huawei.com>
Signed-off-by: Tiezhu Yang <yangtiezhu@loongson.cn>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>

arch/mips/kernel/smp-cps.c

@@ -451,9 +451,6 @@ static int cps_cpu_disable(void)
 	unsigned cpu = smp_processor_id();
 	struct core_boot_config *core_cfg;
 
-	if (!cpu)
-		return -EBUSY;
-
 	if (!cps_pm_support_state(CPS_PM_POWER_GATED))
 		return -EINVAL;