DMAENGINE: COH 901 318 rename confusing vars

This fixes up the code with a lot of comments that make it readable,
rename things with opaque names like "data" into something more
appropriate, and remove some very confusing BUG() statements.

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