MFC r215847:

MFC r215847:
  Disable retrying RX descriptor loading. The counter is used to set
  number of retry to be performed whenever controller found RX
  descriptor was empty. RX empty interrupt is generated only when the
  retry counter is over. Experimentation shows retrying RX descriptor
  loading increased number of dropped frames under flow-control
  enabled environments so disable it and have controller generate RX
  empty interrupt as fast as it can.
  While I'm here fix RXCSR_DESC_RT_CNT macro.

git-svn-id: svn://svn.freebsd.org/base/stable/8@216025 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f