power: qpnp-fg: report zero soc immediately when empty

The empty soc interrupt is a voltage based interrupt that triggers
whenever the battery voltage is below a programmed threshold. This is
intended to be a last line of defense in case the soc count is
inaccurate.

In order to prevent UVLOs, immediately report zero soc whenever the empty
soc is raised.

CRs-Fixed: 756489
Change-Id: Ib3c8cc06efa1a834704fc166a393e9959ed01f0e
Signed-off-by: Xiaozhe Shi <xiaozhes@codeaurora.org>