pre_req() runs dma_map_sg() and prepares the dma descriptor for the next
mmc data transfer. post_req() runs dma_unmap_sg. If not calling pre_req()
before mmci_request(), mmci_request() will prepare the cache and dma just
like it did it before. It is optional to use pre_req() and post_req()
for mmci.
Signed-off-by: Per Forlin <per.forlin@linaro.org> Tested-by: Linus Walleij <linus.walleij@linaro.org> Signed-off-by: Chris Ball <cjb@laptop.org>