mmc: mmci: stm32: make sdmmc_idma_validate_data static

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>

drivers/mmc/host/mmci_stm32_sdmmc.c

@@ -25,8 +25,8 @@ struct sdmmc_priv {
 	void *sg_cpu;
 };
 
-int sdmmc_idma_validate_data(struct mmci_host *host,
-			     struct mmc_data *data)
+static int sdmmc_idma_validate_data(struct mmci_host *host,
+				    struct mmc_data *data)
 {
 	struct scatterlist *sg;
 	int i;