xtensa: update *pos in cpuinfo_op.next

Increment *pos in the cpuinfo_op.next to fix the following warning
triggered by cat /proc/cpuinfo:

  seq_file: buggy .next function c_next did not update position index
Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>

arch/xtensa/kernel/setup.c

@@ -724,7 +724,8 @@ c_start(struct seq_file *f, loff_t *pos)
 static void *
 c_next(struct seq_file *f, void *v, loff_t *pos)
 {
-	return NULL;
+	++*pos;
+	return c_start(f, pos);
 }
 
 static void