Short circuit sdma_txclean() by adding an __sdma_txclean()
that is only called when the tx has sdma mappings.

Convert internal calls to __sdma_txclean().

This removes a call from the critical path.
Reviewed-by: Sebastian Sanchez <sebastian.sanchez@intel.com>
Signed-off-by: Mike Marciniszyn <mike.marciniszyn@intel.com>
Signed-off-by: Dennis Dalessandro <dennis.dalessandro@intel.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>