hwmon: qpnp-adc-voltage: Fix lockup during recurring measurement

The gain and offset is used during final calculation.
These are called during single conversion request or
recurring channel measurement, each of these requests
hold the same lock and can cause a lockup.

Change-Id: I61afe01eae609cdc465721721e19c66da9aeb1e6
Signed-off-by: Siddartha Mohanadoss <smohanad@codeaurora.org>