netX90 PHY: Add a delay between programming of a fixed duplex mode and the PHY fixup routines

Programming the PHY into fixed duplex modes 100FDX and 100HDX sometimes causes no link to be established. It is not entirely clear yet why his is happening, but inserting a delay between the sequence of programming a forced duplex mode and the PHY fixup stuff seems to fix the issue. Add such a delay for now and carefully reconsider the PHY register accesses at a later point in time.

We decided for a 10ms delay since this is also used in the PHY testcode of group SDO.