hwmon: qpnp-adc: fix iadc compensation for pmic 3.0

With PMIC 3.0, the iadc compensation scheme will return 0 current
because of an conversion bug. Fix this in the iadc driver.

Also, the iadc driver was incorrectly assuming GF as the fab. Remove
this from the compensation scheme.

Finally, when the ADC revision is unsupported, simply do not
compensate.

Change-Id: I8d4ec7bfe7e125bd18da9dba5d78f988ffaea515
Signed-off-by: Xiaozhe Shi <xiaozhes@codeaurora.org>