hw/net/lan9118_phy: Add missing 100 mbps full duplex advertisement

hw/net/lan9118_phy: Add missing 100 mbps full duplex advertisement

The real device advertises this mode and the device model already advertises
100 mbps half duplex and 10 mbps full+half duplex. So advertise this mode to
make the model more realistic.

Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Bernhard Beschow <shentey@gmail.com>
Tested-by: Guenter Roeck <linux@roeck-us.net>
Message-id: 20241102125724.532843-6-shentey@gmail.com
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>