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

Add additional modes for specific link speed. Use ethtool APIs to get the
current link speed 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 speed. (example scenario a PHY that have 2 LED
connected one green and one orange where the green is turned on with
1000mbps speed and orange is turned on with 10mpbs speed)

On mode set from sysfs we check if we have enabled split link speed mode
and reject enabling generic link mode to prevent wrong and redundant
configuration.

Rework logic on the set baseline state to support these new modes to
select if we need to turn on or off the LED.

Add additional modes:
- link_10: Turn on LED when link speed is 10mbps
- link_100: Turn on LED when link speed is 100mbps
- link_1000: Turn on LED when link speed is 1000mbps

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>