so we increment the right thing. (All work and not enough parens
make Bill something something...) This makes the RealTek 8139C+
driver work correctly.
Also fix some mtx_lock_spin()s and mtx_unlock_spin()s that should
have been just plain mtx_lock()s and mtx_unlock()s.
In kern_ndis.c: remove duplicate code from ndis_send_packets() and
just call the senddone handler (ndis_txeof()).