ste_dma40: add variable to indicate valid dma_cfg

Since we want to reduce the amount of required channel
configuration and remove channel_type, don't depend on it
to indicate whether the configuration is valid.

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