Merge r207585 from cas(4):

• Don't probe for PHYs if we already know to use a SERDES. Unlike as with cas(4) this only serves to speed up the the device attach though and can only be determined via the OFW device tree but not from the VPD.
• Don't touch the MIF when using a SERDES.
• Add some missing bus space barriers, mainly in the PCS code path.