power: pm8921-bms: iterate rbatt algorithm

When ocv (open circuit voltage of the battery) is adjusted to match a
specific soc, rbatt (the battery resistance) changes too. This is
because rbatt is dependent on the remaining charge which changes as
the ocv changes.

The issue we observed with this changing rbatt was the unusable charge
changes which causes the soc to move away from the soc the algorithm
attempts to match.

To get around this issue adjust both the rbatt and ocv in a iterative
fashion until rbatt value stabilizes.

Change-Id: Id3e36c1a8361a6d71b63fca3e6b0c2a4a9bb7e06
Signed-off-by: Abhijeet Dharmapurikar <adharmap@codeaurora.org>