net: dp83869: Add ability to advertise Fiber connection

Add the ability to advertise the Fiber connection if the strap or the
op-mode is configured for 100Base-FX.

Auto negotiation is not supported on this PHY when in fiber mode.

Signed-off-by: Dan Murphy <dmurphy@ti.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>