power: qpnp-bms: report resistance immediately

Currently, the first calculation of rbatt is ignored. This creates
a period between when the bms power supply is registered and before
the next time SoC is calculated where the resistance value reported
is 0.

Fix this by saving the first calculation of rbatt but not calling
power supply changed until the bms power supply is registered.

Change-Id: I1844defabd9aec8e7a82831f046d66849b5db25b
Signed-off-by: Xiaozhe Shi <xiaozhes@codeaurora.org>