msm: timer: fix missing return value

Change af90f10d "ARM: 6759/1: smp: Select local timers vs broadcast
timer support runtime" missed a return statement, causing a compile
warning:

  arch/arm/mach-msm/timer.c:272: warning: 'return' with no value, in
  function returning non-void

Trivially return 0 for success when running on cpu 0 (to match the
comment and previous behavior).

Signed-off-by: David Brown <davidb@codeaurora.org>