Modify ath_descdma_cleanup() to handle ath_descdma instances with no buffers.
Modify ath_descdma_cleanup() to handle ath_descdma instances with no
buffers.

ath_descdma is now being used for things other than the classical
combination of ath_buf + ath_desc allocations.  In this particular case,
don't try to free and blank out the ath_buf list if it's not passed in.

jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase