Always allocate bwi(4) ring descriptors below the 1GB boundary to give 30bit

Always allocate bwi(4) ring descriptors below the 1GB boundary to give 30bit
devices a chance to work. Use bounce buffers for mbufs on 30bit devices.

This fixes "intr fatal TX/RX" errors that render the internal wifi on many
macppc machines unusable. However, packet loss problems remain. In my testing
the device works fine sometimes, but experiences packet loss rates of up
to 80% at other times. Still, this is a step forward.

Helpful hints from claudio@ and dlg@
Tested on macppc by mpi@ and myself
"go ahead" kettenis@, ok mpi@