ACPI: thinkpad-acpi: make sure DSDT TMPx readings don't return +128

We get +128 instead of -128 from the DSDT TMPx methods, due to errors when
converting a EC byte return that is a s8 to an ACPI handler return that is
an int.

Fix it once and for all, by clamping acceptable temperature readings from
DSDT TMPx so that anything outside the [-127,+127] range is converted to
TP_EC_THERMAL_TMP_NA (-128).

Signed-off-by: Henrique de Moraes Holschuh <hmh@hmh.eng.br>
Cc: Michael Olbrich <michael.olbrich@gmx.net>
Signed-off-by: Len Brown <len.brown@intel.com>