intel_idle: remove use and definition of MWAIT_MAX_NUM_CSTATES

Cosmetic only.

Replace use of MWAIT_MAX_NUM_CSTATES with CPUIDLE_STATE_MAX.
They are both 8, so this patch has no functional change.

The reason to change is that intel_idle will soon be able
to export more than the 8 "major" states supported by MWAIT.
When we hit that limit, it is important to know
where the limit comes from.

Signed-off-by: Len Brown <len.brown@intel.com>