hwmon: qpnp-adc-common: Fix vbatt reverse scaling

Update the scaling function for vbatt reverse scaling
api to fix issues while performing integer operations.
Clients request the voltage in micro-volts and the
reverse scaling fix is the use the correct absolute
reverse scaling function to convert it to voltage thresholds.

Change-Id: Iea61b768daf708b4f59e5728ed1249dd60f18258
Signed-off-by: Siddartha Mohanadoss <smohanad@codeaurora.org>