power: qpnp-fg-gen4: Show 0% SOC when battery voltage is low

Currently, battery voltage is checked only when MSOC_EMPTY bit
is set if the state of charge (SOC) is not showing 0%. Instead,
show 0% when the vbatt_low interrupt trips and battery voltage
is lower than cutoff voltage. With this, is_batt_empty() is not
needed anymore. Remove it.

Change-Id: Id5d5ba06d1d22827e39dd4dd757f8a123b2fa628
Signed-off-by: Subbaraman Narayanamurthy <subbaram@codeaurora.org>