mmc: sdhci: use dma_map_single() instead of flush_cache() before DMA

Currently, sdhci_prepare_dma() calls flush_cache() regardless of the
DMA direction.

Actually, cache invalidation is enough when reading data from the device.

This is correctly handled by dma_map_single(), which mimics the DMA-API
in Linux kernel. Drivers can be agnostic which cache operation occurs
behind the scene.

This commit also sanitizes the difference between the virtual address
and the dma address.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>