hwmon: qpnp-adc-current: Fix IADC current units

At low current values, not using the correct units
for scaling the voltage and dividing it by the appropriate
units for rsense causes incorrect resolution in units for
current. Fix it by using the correct units.

Change-Id: Ia76382268b69db194f5f17f5a129140e75d66298
Signed-off-by: Siddartha Mohanadoss <smohanad@codeaurora.org>