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.
MFC: r247570, r247591

- Make tables, device ID strings etc const. This includes #ifdef'ing 0
  aac_command_status_table, which is actually unused since r111532.
  While at it, make aac_if a pointer to the now const interface tables
  instead of copying them over to the softc (this alone already reduces the
  size of aac.ko on amd64 by ~1 KiB).
- Remove redundant softc members.
- Use DEVMETHOD_END.
- Use NULL instead of 0 for pointers.
- Remove redundant bzero(9)'ing of the softc.
- Use pci_enable_busmaster(9) instead of duplicating it.
- Remove redundant checking for PCIM_CMD_MEMEN (resource allocation will
  just fail).
- Canonicalize the error messages in case of resource allocation failures.
- Add support for using MSI instead of INTx, controllable via the tunable
  hw.aac.enable_msi (defaulting to on).