power: pm8921-bms: fix rounding down state of charge

The code is rounding down state of charge value in the final step of the
calculation. Instead use a division that returns the closest integer.

Change-Id: Ia47d1c4eb684dd4c31e454dc762add0feab66b75
Signed-off-by: Abhijeet Dharmapurikar <adharmap@codeaurora.org>