power: supply: generic-adc-battery: use simple-battery API

Constant battery data is available through power-supply's simple-battery
API. This works automatically, so the manual handling can be removed
without loosing any feature :)

Note, that the POWER_SUPPLY_STATUS_FULL check for the level variable can
be dropped, since the variable is never written. It can be re-introduced
properly once the driver gets functionality to calculate the current
charge level. Apart from that the check must be done fuzzy anyways,
since charge estimation usually is not precise enough to always return
exactly the full charge capacity for a full battery.

Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Reviewed-by: Matti Vaittinen <mazziesaccount@gmail.com>
Signed-off-by: Sebastian Reichel <sre@kernel.org>