leds-gpio: Implement the brightness_get method

The power LED uses some clever logic that means it is driven
by a voltage measuring circuit when configured as input, otherwise
it is driven by the GPIO output value. This patch wires up the
brightness_get method for leds-gpio so that user-space can monitor
the LED value via /sys/class/gpio/led1/brightness. Using the input
trigger this returns an indication of the system power health,
otherwise it is just whatever value the trigger has written most
recently.

See: #1064

drivers/leds/leds-gpio.c

@@ -83,6 +83,13 @@ static void gpio_led_set(struct led_classdev *led_cdev,
  }
 }
 
+static enum led_brightness gpio_led_get(struct led_classdev *led_cdev)
+{
+ struct gpio_led_data *led_dat =
+ container_of(led_cdev, struct gpio_led_data, cdev);
+ return gpiod_get_value_cansleep(led_dat->gpiod) ? LED_FULL : LED_OFF;
+}
+
 static int gpio_blink_set(struct led_classdev *led_cdev,
  unsigned long *delay_on, unsigned long *delay_off)
 {
@@ -139,6 +146,7 @@ static int create_gpio_led(const struct gpio_led *template,
  led_dat->cdev.blink_set = gpio_blink_set;
  }
  led_dat->cdev.brightness_set = gpio_led_set;
+ led_dat->cdev.brightness_get = gpio_led_get;
  if (template->default_state == LEDS_GPIO_DEFSTATE_KEEP)
  state = !!gpiod_get_value_cansleep(led_dat->gpiod);
  else