power: supply: ab8500: Standardize termination current

The AB8500 custom termination current can be replaced by the
corresponding struct power_supply_battery_info field.

Remove the struct member and amend the code to use the
standard property.

Add *_ua suffix for clarity and to make sure we have
changed all code sites using this member.

Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>