Be more like the BGE driver.  Ensure that at least 16 TX descriptors are kept unused in the ring.  This check should probably be moved up to bce_start_locked at some point, as it'll make the loop up there slightly more efficient, and will eliminate a costly set of busdma operations when the ring is full.  But this works for now.
Be more like the BGE driver.  Ensure that at least 16 TX descriptors are
kept unused in the ring.  This check should probably be moved up to
bce_start_locked at some point, as it'll make the loop up there slightly
more efficient, and will eliminate a costly set of busdma operations when
the ring is full.  But this works for now.

This makes all of my UDP torture tests work.  I'll cautiously say that
it might even work for other users now.  Feedback is appreciated.

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