hwmon: qpnp-adc-common: Fix setting reverse vbatt values

Use int64_t type to while scaling the threshold values with
absolute calibration arithmetic avoid overflow errors.

Change-Id: I29d3b4ad71776a45ec1cd2c076b0f058b7bf8723
Signed-off-by: Siddartha Mohanadoss <smohanad@codeaurora.org>