dm: mmc: Update mmc_get_mmc_dev() to use const *

This function does not modify the device to change it to use const *, so
that callers with a const udevice * can call it without a cast.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Jaehoon Chung <jh80.chung@samsung.com>
diff --git a/drivers/mmc/mmc-uclass.c b/drivers/mmc/mmc-uclass.c
index cb26d841..f313bc1 100644
--- a/drivers/mmc/mmc-uclass.c
+++ b/drivers/mmc/mmc-uclass.c
@@ -239,7 +239,7 @@
 	return 0;
 }
 
-struct mmc *mmc_get_mmc_dev(struct udevice *dev)
+struct mmc *mmc_get_mmc_dev(const struct udevice *dev)
 {
 	struct mmc_uclass_priv *upriv;