DMAENGINE: ste_dma40: deny ops on non-alloc channels

Added checks to deny operating on none-allocated channels.

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>