power: qpnp-bms: Use charger's battery insertion OCV

Use the charger driver's OCV reading taken upon battery insertion
instead of estimating OCV when the BMS driver detects a new battery.

This drastically improves the accuracy of battery state of charge
when hotswapping.

Change-Id: Iacf17370f0d5d9c422180f08de28730e4bd66d18
Signed-off-by: Xiaozhe Shi <xiaozhes@codeaurora.org>