platform-x86: intel_mid_thermal: turn off thermistor voltage by default

Instead of complaining that the voltage is on, we can just ask the MSIC to
turn the voltage off. This should save some power.

Voltage for thermistors is turned on when ADC conversion is initiated.

Signed-off-by: Mika Westerberg <mika.westerberg@linux.intel.com>
Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Signed-off-by: Alan Cox <alan@linux.intel.com>
Signed-off-by: Matthew Garrett <mjg@redhat.com>