Use ADDR64 opcodes to specify 64bit DMA addresses for all buffers. This

Use ADDR64 opcodes to specify 64bit DMA addresses for all buffers.  This
consumes two ring entries per segment, halving the effective size of the
rings, but keeps things simple.  This is required for the onboard nic
on the Overdrive 1000 to work.

ok kettenis@, tested on i386 by jsg@