cpu: Fix W=1 build kernel-doc warning

cpu: Fix W=1 build kernel-doc warning

Building the kernel with W=1 generates the following warning:

  kernel/cpu.c:2693: warning: This comment starts with '/**',
  		     but isn't a kernel-doc comment.

The function topology_is_core_online() is a simple helper function and
doesn't need a kernel-doc comment.

Use a normal comment instead.

Signed-off-by: Thorsten Blum <thorsten.blum@toblux.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/all/20240825221152.71951-2-thorsten.blum@toblux.com