power: gpio-charger: do not use gpio value directly

Some gpio implementations return interesting values for gpio_get_value when
the value is not 0 - as seen on a imx6sl board. Therefore do not use the
value returned from gpio_get_value directly but simply check for 0 or not 0.

Signed-off-by: Heiko Stuebner <heiko.stuebner@bq.com>
Reviewed-by: Doug Anderson <dianders@chromium.org>
Tested-by: Doug Anderson <dianders@chromium.org>
Signed-off-by: Sebastian Reichel <sre@kernel.org>