power: pm8921-bms: fix shutting down early at cold temperature

At cold temperatures the battery resistance is high. This high resistance
translates to a high unusable charge(UUC) value and causes early 0% state
of charge report. The phone survives for only about 20 minutes from a fully
charged state.

Fix this by slowly increasing/decreasing the UUC value as the battery
discharges/charges respectively. Remember the last uuc reported and that
forms the base from which we change it in steps.

Change-Id: I9c09bc8274de1c7b1a1fcae586653dd9e91e161f
Signed-off-by: Abhijeet Dharmapurikar <adharmap@codeaurora.org>