power: qpnp-fg-gen4: Use ESR_ACT for available power calculation

As per the hardware recommendation, use ESR_ACT to calculate the
available power instead of ESR for better accuracy.

Change-Id: Ie21b3522a0aeb5047959457b0fbf1f9c8b185f29
Signed-off-by: Subbaraman Narayanamurthy <subbaram@codeaurora.org>