Document now hidable link speed modes for the LED netdev trigger.

Link speed modes are now showed only if the named network device
supports them and are hidden if not.
Signed-off-by: Christian Marangi <ansuelsmth@gmail.com>
Reviewed-by: Marek Behún <kabel@kernel.org>
Link: https://lore.kernel.org/r/20240111160501.1774-2-ansuelsmth@gmail.comSigned-off-by: Lee Jones <lee@kernel.org>