power: pm8921-charger: calibrate hkadc upon insertion/removal

The hkadc should be calibrated every time a charger is plugged in
or plugged out. This is because the reference voltage shifts a bit
when a charger is plugged in.

Call the bms api to calibrate hkadc.

Change-Id: I89564dcc3ef32af2c3abe422b901c499e91fd2d5
Signed-off-by: Abhijeet Dharmapurikar <adharmap@codeaurora.org>