power: pm8921-bms: fix hkadc readings

The procedure to calculate the battery voltage from xoadc raw readings
needs to be updated. Currently the driver multiplies the readings by 3
before calibration. This step needs to be done after calibration.

Also update all battery voltage based read_xxxx functions to
return the calibrated value.

Additionally call the hkadc calibration function at probe time
so early readings of battery voltage will be accurate.

Change-Id: I19c4ae8f124f3442ddffd651445db529ceac1cfe
Signed-off-by: Abhijeet Dharmapurikar <adharmap@codeaurora.org>