MFC: if_gem.c 1.37, 1.39, 1.40, 1.43; if_gemvar.h 1.12

MFC: if_gem.c 1.37, 1.39, 1.40, 1.43; if_gemvar.h 1.12

- Fix -Wundef warnings found when compiling i386 LINT, GENERIC and
  custom kernels.
- Use bus_get_dma_tag() so gem(4) works on platforms requiring it.
- Use the gem_tick() callout instead of if_slowtimo() for driving
  gem_watchdog() in order to avoid races accessing if_timer.
  While at it relax the watchdog a bit by reloading it in gem_tint()
  if there are still packets enqueued.
- Don't bother to set if_mtu to ETHERMTU, ether_ifattach() does that.
- Fix inconsistencies in prototypes.