x86, mrst: make mrst_timer_options an enum

We have an enum mrst_timer_options, use it so that the kernel knows if
we're missing something from a switch statement or equivalent.

Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>
LKML-Reference: <1274295685-6774-4-git-send-email-jacob.jun.pan@linux.intel.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Jacob Pan <jacob.jun.pan@linux.intel.com>