x86, mrst: make mrst_identify_cpu() an inline returning enum

We have an enum, might as well use it.  While we're at it, make it an
inline... there is really no point in calling a function for this
stuff.

LKML-Reference: <1274295685-6774-3-git-send-email-jacob.jun.pan@linux.intel.com>
Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>
Acked-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Jacob Pan <jacob.jun.pan@linux.intel.com>