net: phy: phy_device: Call into the PHY driver to set LED brightness

net: phy: phy_device: Call into the PHY driver to set LED brightness

Linux LEDs can be software controlled via the brightness file in /sys.
LED drivers need to implement a brightness_set function which the core
will call. Implement an intermediary in phy_device, which will call
into the phy driver if it implements the necessary function.

Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Christian Marangi <ansuelsmth@gmail.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>