power: qpnp-fg-gen4: Use ESR_ACTUAL for calculating cell impedance

Cell impedance calculation is currently getting ESR which is used
for TTF calculations. Instead, use ESR_ACTUAL which provides a
stable value as per the hardware recommendation.

Change-Id: I2e32a209287134869f6e4e7e012626e48d64b09a
Signed-off-by: Subbaraman Narayanamurthy <subbaram@codeaurora.org>