Disable setting the MII port speed.

Disable setting the MII port speed.

This seems to break at least my test board here (AR71xx + AR8316 switch
PHY).  Since I do have a whole sleuth of "normal" PHY boards (with
an AR71xx on a normal PHY port), I'll do some further testing with those
to determine whether this is a general issue, or whether it's limited
to the behaviour of the "fake" dedicated PHY port mode on these atheros
switches.