leds: trigger: netdev: add additional specific link duplex mode

Add additional modes for specific link duplex. Use ethtool APIs to get the
current link duplex and enable the LED accordingly. Under netdev event
handler the rtnl lock is already held and is not needed to be set to
access ethtool APIs.

This is especially useful for PHY and Switch that supports LEDs hw
control for specific link duplex.

Add additional modes:
- half_duplex: Turn on LED when link is half duplex
- full_duplex: Turn on LED when link is full duplex

Signed-off-by: Christian Marangi <ansuelsmth@gmail.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Acked-by: Lee Jones <lee@kernel.org>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>