DMAENGINE: COH 901 318 cleanups

This cleans up the some debug code that was not working in the
COH 901 318 driver, adds some helpful comments and rearrange the
code a bit.

Signed-off-by: Linus Walleij <linus.walleij@stericsson.com>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>