net: phy: bcm54xx: Encode link speed and activity into LEDs

Previously the green and amber LEDs on this quad PHY were solid, to
indicate an encoding of the link speed (10/100/1000).

This keeps the LEDs always on just as before, but now they flash on
Rx/Tx activity.

Signed-off-by: Vladimir Oltean <olteanv@gmail.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>