qpnp-fg-gen3: Show a fake SOC and voltage when battery is missing

During battery hotswap, a strong charger will be supplying enough
current to keep the device up and running. Currently, FG driver
shows up the monotonic SOC and voltage as-is (which is zero) to
the battery power supply and hence to the userspace.

Show a fake battery SOC (50%) and fake battery voltage (3.7 V)
when the battery is missing. If a strong charger is not present,
then the device will be powered down anyways.

Change-Id: Ia2c85d1e75bbf0c937a7e36e786362c473c7f7cd
Signed-off-by: Subbaraman Narayanamurthy <subbaram@codeaurora.org>