power: supply: improve function-level documentation

In drivers/power/supply/ab8500_fg.c, fix two typos and adjust the function
name in two cases to correspond to the names of the defined functions.

In drivers/power/supply/power_supply_core.c, change two variable names to
the names of the corresponding parameters.

Issue detected using Coccinelle (http://coccinelle.lip6.fr/

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Signed-off-by: Julia Lawall <Julia.Lawall@lip6.fr>
Signed-off-by: Sebastian Reichel <sre@kernel.org>