hwmon: qpnp-current: Use ideal gain for PM8026

The IADC on PM8026 has been found to have improved accuracy if the
gain compensation is removed from the conversion equation. Implement
this in the IADC driver by faking a gain read of offset + ideal gain
during calibration.

Change-Id: I85abf713e796666946674e5ae603c918cfa9ae5a
Signed-off-by: Xiaozhe Shi <xiaozhes@codeaurora.org>