DMAENGINE: ste_dma40: fix bug related to callback handling

The callback got called even when it was not supposed to. Also
removed some not needed interrupt trigger on/off code.

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>