DMAENGINE: ste_dma40: use kmem cache

Use kmem cache instead of own cache, reducing code duplication
and bug sources.

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>