mmc: renesas_sdhi: use helpers to access struct scatterlist members

Instead of directly accessing the members of struct scatterlist use the
helpers mmc_get_dma_dir() and sg_dma_address() in
renesas_sdhi_internal_dmac_start_dma(). Based on previous work by
Masaharu Hayakawa.

Signed-off-by: Niklas Söderlund <niklas.soderlund+renesas@ragnatech.se>
[rebased to mmc/next]
Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Reviewed-by: Simon Horman <horms+renesas@verge.net.au>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>