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.