ACPI: ibm-acpi: Use a enum to select the thermal sensor reading strategy

This patch consolidades all decisions regarding the strategy to be used to
read thinkpad thermal sensors into a single enum, and refactors the
thermal sensor reading code to use a much more readable (and easier to
extend) switch() construct, in a separate function.

Signed-off-by: Henrique de Moraes Holschuh <hmh@hmh.eng.br>