cpuidle: menu: Tweak threshold use in get_typical_interval()

To prepare get_typical_interval() for subsequent changes, rearrange
the use of the data point threshold in it a bit and initialize that
threshold to UINT_MAX which is more consistent with its data type.

No intentional functional impact.

Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Tested-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>
Reviewed-by: Christian Loehle <christian.loehle@arm.com>
Tested-by: Christian Loehle <christian.loehle@arm.com>
Tested-by: Aboorva Devarajan <aboorvad@linux.ibm.com>
Link: https://patch.msgid.link/8490144.T7Z3S40VBb@rjwysocki.net