mmc: mmci: Don't share un-implemented DMA functions

Some of the DMA functions are shared via mmci.h, however they are not
implemented unless CONFIG_DMA_ENGINE is set. Therefore, add that constraint
to the header file as well.

Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>
Acked-by: Linus Walleij <linus.walleij@linaro.org>