DMAENGINE: ste_dma40: removed a few magic numbers

Make sure to extract the revision field explicitly and document
what bits are being accessed here without magic numbers.

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