hwmon: qpnp-adc: Fix potential errors

There is a possibility of potential out of bound array
access errors and uninitialized variable access from code
inspection. Add additional checks in voltage ADC and
TSENS to check for such errors.

Change-Id: I1213cd76fb636aa457c88d3bdc3740b692f01cce
Signed-off-by: Siddartha Mohanadoss <smohanad@codeaurora.org>