Fix another bug introduced in r212109. We should unload DMA maps only after sending the last fragment of a frame so the mbuf pointer also should be stored in the last descriptor index.
Fix another bug introduced in r212109. We should unload DMA maps
only after sending the last fragment of a frame so the mbuf pointer
also should be stored in the last descriptor index.

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