mmc: mmci_sdmmc: Replace sg_dma_xxx macros

sg_dma_xxx should be used after a dma_map_sg call has been done to get bus
addresses of each of the SG entries and their lengths.  But mmci_host_ops
validate_data can be called before dma_map_sg.  This patch replaces theses
macros by sg->offset and sg->length which are always defined.

Signed-off-by: Ludovic Barre <ludovic.barre@st.com>
Link: https://lore.kernel.org/r/20200128090636.13689-2-ludovic.barre@st.com
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>