Modify the if_arge code to use a /fixed/ media mode when it's configured.

Otherwise, the initial media speed would change if a PHY is hooked up,
sending PHY speed notifications. For the AP135 at least, the RGMII
PHY has a static speed/duplex configured and if the PHY plumbing
attaches the PHY to the if_arge interface, the first link speed change
from 1000/full will set the MAC to something that isn't useful.

This shouldn't affect any other platforms - everything I looked at is
using hard-coded speed/duplex as static, as they're facing a switch
with no PHY attached.