qpnp-fg-gen3: Linearize SOC during discharging in SOC masking algorithm

During SOC masking, SOC 100 will be shown to the user after the
charge termination and as long as the charger is inserted. After
the charger is removed or device is discharging when the charger
was inserted, showing the real monotonic SOC instantly reduces
the user experience. Linearize the drop in SOC so that it can be
spread to a wider range.

Change-Id: I509e59f8e5d9e9addd8c5721369fbcf664158338
Signed-off-by: Subbaraman Narayanamurthy <subbaram@codeaurora.org>