MFC r221565-221568,221579:

MFC r221565-221568,221579:
r221565:
  Reuse the TX descriptor(DPD) if xl_encap() failed instead of just
  picking the next available one. This may explain why xl(4) sees TX
  underrun error with no queued frame. I hope this addresses a long
  standing xl(4) watchdog timeout issue as well.

  Obtained from:        OpenBSD

r221566,221579:
  Rename xl_stats_update() callout handler to xl_tick() and move MII
  tick driving logic to xl_tick(). Now xl_tick() handles MII tick as
  well as periodic updating of statistics.
  This change removes a hack used in interrupt handler where it
  wanted to update statistics without driving MII tick.

r221567:
  Rearm watchdog timer if driver kick controller to recover from TX
  underrun error.
  While here, prepend 0x to status code to show TX status is hex
  number.

r221568:
  XL_DMACTL is 32bit register, use 32bit write macro.
  While I'm here add more bits for the register.

git-svn-id: svn://svn.freebsd.org/base/stable/8@222387 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f