power: qpnp-bms: add adjust_soc for discharging

Fill in the adjust_soc function to provide more linear soc
while discharging.

Change-Id: I2c6cb16f2ad627a9f3d1e3b8802259b664eb2c99
Signed-off-by: Xiaozhe Shi <xiaozhes@codeaurora.org>