power: pm8921-bms: reintroduce hkadc calibration during probe

The hkadc calibration process takes only one sample. This should be
a quick procedure to be done during probe. Without calibration the
voltage readings are skewed which affect the accuracy of BMS.

Change-Id: Ibd1020113de05e8214d41ffb975d4c95d3bb1ccf
Signed-off-by: Abhijeet Dharmapurikar <adharmap@codeaurora.org>