power: pm8921-bms: use accurate LSB voltage for calibration channels

The driver uses 97.7mV as the value of a single bit for calibration
channels. It should however use 97.656mV for these channels.

These fixes a few mV error seen in OCV(open circuit voltage) readings.

Also, remove the sanity checks that forces voltage to be in a certain
range. Forcing voltage on calibration channels could cause errors in
battery voltage reporting.

Change-Id: I21cc665a1b11597692968d4c96ba7f838402c74f
Signed-off-by: Abhijeet Dharmapurikar <adharmap@codeaurora.org>