MFC: r213364

MFC: r213364

- In the spirit of previous simplifications factor out the checks for a
  different PHY instance being selected and isolation out into the wrappers
  around the service methods rather than duplicating them over and over
  again (besides, a PHY driver shouldn't need to care about which instance
  it actually is).
- Centralize the check for the need to isolate a non-zero PHY instance not
  supporting isolation in mii_mediachg() and just ignore it rather than
  panicing, which should sufficient given that a) things are likely to
  just work anyway if one doesn't plug in more than one port at a time and
  b) refusing to attach in this case just leaves us in a unknown but most
  likely also not exactly correct configuration (besides several drivers
  setting MIIF_NOISOLATE didn't care about these anyway, probably due to
  setting this flag for no real reason).
- Minor fixes like removing unnecessary setting of sc->mii_anegticks,
  using sc->mii_anegticks instead of hardcoded values etc.

git-svn-id: svn://svn.freebsd.org/base/stable/8@214266 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f