hrtimer: Use a bullet for the returns bullet list

That gets rid of this warning:

   ./kernel/time/hrtimer.c:1119: WARNING: Block quote ends without a blank line; unexpected unindent.

and displays nicely both at the source code and at the produced
documentation.
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Linux Doc Mailing List <linux-doc@vger.kernel.org>
Cc: Mauro Carvalho Chehab <mchehab@infradead.org>
Cc: Jonathan Corbet <corbet@lwn.net>
Link: https://lkml.kernel.org/r/74ddad7dac331b4e5ce4a90e15c8a49e3a16d2ac.1561372382.git.mchehab+samsung@kernel.org

kernel/time/hrtimer.c

@@ -1114,9 +1114,10 @@ EXPORT_SYMBOL_GPL(hrtimer_start_range_ns);
  * @timer:	hrtimer to stop
  *
  * Returns:
- *  0 when the timer was not active
- *  1 when the timer was active
- * -1 when the timer is currently executing the callback function and
+ *
+ *  *  0 when the timer was not active
+ *  *  1 when the timer was active
+ *  * -1 when the timer is currently executing the callback function and
  *    cannot be stopped
  */
 int hrtimer_try_to_cancel(struct hrtimer *timer)