power: pm8921-bms: increase/decrease UUC 1% at a time

We see huge jumps in Unusable Charge (UUC) as temperature, average
load or state of charge (SOC) changes, which results in huge jumps
in the reported capacity.

Increase/Decrease UUC slowly (limit 1%) each time SOC is requested.
This results in linearity improvements.

Change-Id: I82022aa499a53c5ea42e7079e96d3c94dfc0be01
Signed-off-by: Abhijeet Dharmapurikar <adharmap@codeaurora.org>