DMAENGINE: define a dummy filter function for ste_dma40

All platform data has to be made conditional on CONFIG_STEDMA40
or we can provide a simple dummy filter functions as to avoid
cluttering the code with other #ifdef:s.

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