thermal: ti-bandgap: Fix adc value datatype

The CORE_TEMP_SENSOR_MPU register gives a raw adc value which needs to
be indexed into a lookup table to get the actual temperature. Fix the
naming and datatype of the adc value variable.

Signed-off-by: Faiz Abbas <faiz_abbas@ti.com>
Signed-off-by: Lokesh Vutla <lokeshvutla@ti.com>