mmc: tmio_mmc: use dmaengine helpers, drop submit check

Use the new dmaengine helper functions, and drop the error check
on the returned cookier from the dmaengine - we recently
established that this is really not allowed to fail.

Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Chris Ball <cjb@laptop.org>