Fix kernel-doc warning, add @timer description:

  Warning(kernel/timer.c:335): No description found for parameter 'timer'
Signed-off-by: NRandy Dunlap <randy.dunlap@oracle.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>