mmc: mmci: Re-work code starting DMA for the qcom variant

Having mmci_dmae_start() to invoke the shared function, dml_start_xfer(),
explicitly for the qcom variant isn't very nice. Let's clean up this code
by moving the qcom specific parts into the qcom ->dma_start() callback and
then drop dml_start_xfer() altogether.

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