vge_encap() should bail out if there is not enough free
TX descriptor _OR_ TX descriptor is still owned by the chip.
Anyway, we already check if the TX descriptor already has an mbuf
to be sent in vge_start() so no need to bother to check sc_tx_free
and VGE_TDSTS_OWN in the descriptor in normal case, then make it use
KASSERT(9) or wrap with #ifdef DIAGNOSTIC.
XXX1: I don't know why original FreeBSD driver checks
if a number of free TX descriptors is more than two, not zero.
XXX2: Is it better to check a number of free descriptors in vge_start()
like other our drivers rather than mbuf chain for each descriptor?
c04f54e8f4