power: pm8921-bms: remove Dynamic Unusable Charge algorithm

The dynamic UUC algorithm changes the UUC as load, state of charge and
temperature changes. This uncontrolled behaviour causes unacceptable
jumps in state of charge numbers.

Replace the dynamic UUC algorithm with a simple average current based
UUC. The average current is calculated by remembering the load for last
few (16) samples. Also to maintain a reasonable UUC while charging, a
load of 300mA is assumed.

Note that the first time UUC is calculated we don't have load samples
and in that case the instantaneous current is used.

Since we now don't change the UUC with respect to max possible load
(itest), the usage of this value is removed.  Also instead of failure
voltage we introduce cutoff voltage which represents the loaded voltage
by which the battery should be reported 0%.

Change-Id: Ia640164ee2c9690537308d4e840953824ba15b58
Signed-off-by: Abhijeet Dharmapurikar <adharmap@codeaurora.org>