- Encapsulate the code responsible for initializing a new TX descriptor

- Encapsulate the code responsible for initializing a new TX descriptor
  from an mbuf into the fxp_encap() function, as done in other drivers.
- Don't waste time calling bus_dmamap_load_mbuf() if we know the mbuf
  chain is too long to fit in a TX descriptor, call m_defrag() first.
- Convert fxp(4) to use bus_dmamap_load_mbuf_sg().