power: pm8921-bms: use shutdown soc if it is within limits

When a strong battery is removed, it was seen that the battery voltage
lines on the phone take about five to six seconds to go below 2.1volts
where the pmic resets all the battery backed registers.

If a new battery is plugged in within this time the driver will force
the shutdown soc on this battery which is incorrect.

Compare the shutdown soc with the calculated soc and if they are
different than a configurable limit, simply discard the shutdown soc
and use the calculated soc.

Change-Id: I02e7c78eb5e9df0127ce7e78b0bd9792a8141039
Signed-off-by: Abhijeet Dharmapurikar <adharmap@codeaurora.org>