Make sdmmc_idma_validate_data to avoid the following warning
from sparse:

drivers/mmc/host/mmci_stm32_sdmmc.c:28:5: warning: symbol 'sdmmc_idma_validate_data' was not declared. Should it be static?
Signed-off-by: Ben Dooks <ben.dooks@codethink.co.uk>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>