leds: add API to get attached device for LED hw control

Some specific LED triggers blink the LED based on events from a device
or subsystem.
For example, an LED could be blinked to indicate a network device is
receiving packets, or a disk is reading blocks. To correctly enable and
request the hw control of the LED, the trigger has to check if the
network interface or block device configured via a /sys/class/led file
match the one the LED driver provide for hw control for.

Provide an API call to get the device which the LED blinks for.
Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Christian Marangi <ansuelsmth@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/linux/leds.h

@@ -220,6 +220,12 @@ struct led_classdev {
 	 */
 	int			(*hw_control_get)(struct led_classdev *led_cdev,
 						  unsigned long *flags);
+	/*
+	 * Get the device this LED blinks in response to.
+	 * e.g. for a PHY LED, it is the network device. If the LED is
+	 * not yet associated to a device, return NULL.
+	 */
+	struct device		*(*hw_control_get_device)(struct led_classdev *led_cdev);
 #endif
 
 #ifdef CONFIG_LEDS_BRIGHTNESS_HW_CHANGED