r8169: improve checking for valid LED modes

r8169: improve checking for valid LED modes

After 3a2746320403 ("leds: trigger: netdev: Display only supported link
speed attribute") the check for valid link modes can be simplified.
In addition factor it out, so that it can be re-used by the upcoming
LED support for RTL8125.

Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Link: https://lore.kernel.org/r/8876a9f4-7a2d-48c3-8eae-0d834f5c27c5@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>