power: supply: axp288_fuel_gauge: Rework get_status()

Relying on the (dis)charge current reporting for reporting FULL back to
userspace does not work really well and often leads to the reported status
getting stuck at e.g. 98/99% (the fuelgauge is not perfect) for hours.

What happens is that when the battery is full the axp288 keeps charging it
with a very low current. Until it is really really full and once really
really full, some inaccuracies in the adc lead to it then sometimes
reporting a small discharging rate, even though an external pwr source is
used. So we end up with a status of "charging" for hours after the battery
is actually already full and sometimes this then flip-flops to discharging.

This commit fixes this by first checking if a valid Vbus is present and if
it is present using the fuel-gauge's reported percentage to check for a
full battery.

This commit also changes how get_status() determines if the battery is
charging or discharging when not reporting it as full. We still use the
current direction for this, but instead of reading 4 extra registers for
this (2 16 bit regs), simplify things by using the current-direction bit
in the power-status register, which already gets read anyways.

This also reduces the amount of i2c reads to 1 when on battery and 2
when a valid Vbus is present.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Reviewed-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.co.uk>

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