mmc: sdhci: fix an issue of mixing different types

Fix an issue reported by sparse, since mixed types of parameters are
used on calling dmaengine_prep_slave_sg().

Fixes: 36e1da441fec (mmc: sdhci: add support for using external DMA devices)
Reported-by: kbuild test robot <lkp@intel.com>
Signed-off-by: Chunyan Zhang <zhang.chunyan@linaro.org>
Reviewed-by: Nathan Chancellor <natechancellor@gmail.com>
Acked-by: Adrian Hunter <adrian.hunter@intel.com>
Link: https://lore.kernel.org/r/20200120033223.897-1-zhang.chunyan@linaro.org
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>