MFC: r245926, r245931

MFC: r245926, r245931

- Improve some comments.
- Make bge_lookup_{rev,vendor}() static.
- Factor out chip identification rather than duplicating the code.
- Sanitize bge_probe() a bit (don't hardcode buffer sizes, allow
  bge_lookup_vendor() to return NULL so the excessive panic() can
  be removed there, etc.) and return BUS_PROBE_DEFAULT rather than
  hardcoding 0.
- According to the Linux tg3 driver, BCM57791 and BCM57795 aren't
  capable of Gigabit Ethernet.
- Check the return value of taskqueue_start_threads().
- Mention NetLink controllers in the fallback description, too.

git-svn-id: svn://svn.freebsd.org/base/stable/9@251068 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f