qpnp-fg-gen3: improve TTF accuracy towards EOC

Currently the TTF is inaccurate near 100% SOC. Adjust the tau scale near
100% SOC and adjust the target full battery current to compensate for
the inaccuracy.

Change-Id: I2f6327fab7b71108a928e8c99107522c73c35e07
Signed-off-by: Nicholas Troast <ntroast@codeaurora.org>