leds: gpio: Set max brightness to 1

GPIO LEDs only know 2 states: ON or OFF and do not have PWM capabilities.
However, the max brightness is reported as 255.

This patch sets the max brightness value of a GPIO controlled LED to 1.

Tested on my PinePhone 1.2.

Signed-off-by: Dylan Van Assche <me@dylanvanassche.be>
Signed-off-by: Pavel Machek <pavel@ucw.cz>