power: pm8921-bms: use shutdown_soc to force power on soc

Experiments have shown that shutdown_soc is very accurate. However
upon a restart, the battery may be unsettled, leading to a significantly
different power on soc.

Update the code to force the value of shutdown soc on the power on
soc. To do this simply update the power on OCV (open circuit voltage)
value.

Change-Id: I1015b800e559cb08422364ffc8c8b981ddd4f07e
Signed-off-by: Abhijeet Dharmapurikar <adharmap@codeaurora.org>