MFC r294032:

MFC r294032:

  Fix the handling of the "PDC write transfer length" erratum for at91.  The
  problem affects revision 1xx hardware as well as later versions.  Also, the
  recommended workaround is to set the PDC count register for a 12-byte
  transfer when the actual size is less than that, but there is no need to
  extend or zero-out the data buffer, because the blklen register contains
  the real transfer size and only that many bytes will be transferred.

  Also add a sysctl to turn debugging printfs on or off on the fly.

git-svn-id: svn://svn.freebsd.org/base/stable/10@294687 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f