power: supply: bq27xxx: Move charge reading out of update loop

Most of the functions that read values return a status and put the value
itself in an a function parameter. Update charge reading to match.

As charge state is not checked for changes as part of the update loop,
remove the read of this from the periodic update loop. This saves
I2C/1W bandwidth. It also means we do not have to cache it, fresh
values are read when requested.

Signed-off-by: Andrew Davis <afd@ti.com>
Link: https://lore.kernel.org/r/20240325203129.150030-3-afd@ti.com
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>