powerpc: Simplify cpumask api usage for cpuinfo display

- cpumask_next can take -1 as valid argument. So simplify cpuinfo
  iterator.

- Use cpumask_last to find if this_cpu is last online CPU.

/proc/cpuinfo shows same info with patch.

Reviewed-by: Yury Norov <ynorov@nvidia.com>
Signed-off-by: Shrikanth Hegde <sshegde@linux.ibm.com>
Reviewed-by: Yury Norov <ynorov@gmail.com>
Signed-off-by: Madhavan Srinivasan <maddy@linux.ibm.com>
Link: https://patch.msgid.link/20260427044715.559137-3-sshegde@linux.ibm.com

Author: Shrikanth Hegde

arch/powerpc/kernel/setup-common.c

@@ -323,18 +323,15 @@ static int show_cpuinfo(struct seq_file *m, void *v)
 	seq_putc(m, '\n');
 
 	/* If this is the last cpu, print the summary */
-	if (cpumask_next(cpu_id, cpu_online_mask) >= nr_cpu_ids)
+	if (cpu_id == cpumask_last(cpu_online_mask))
 		show_cpuinfo_summary(m);
 
 	return 0;
 }
 
 static void *c_start(struct seq_file *m, loff_t *pos)
 {
-	if (*pos == 0)	/* just in case, cpu 0 is not the first */
-		*pos = cpumask_first(cpu_online_mask);
-	else
-		*pos = cpumask_next(*pos - 1, cpu_online_mask);
+	*pos = cpumask_next(*pos - 1, cpu_online_mask);
 	if ((*pos) < nr_cpu_ids)
 		return (void *)(unsigned long)(*pos + 1);
 	return NULL;