spl: mmc: Fix subsequent calls to spl_mmc_load with CONFIG_BLK

MMC devices do not have uclass platdata containing blk_descs, only their
child block devices do. Fortunately, we have a function just for this
purpose. This fixes subsequent calls to spl_mmc_load.

Fixes: bf28d9a6599 ("spl: mmc: Use correct MMC device when loading image")
Signed-off-by: Sean Anderson <seanga2@gmail.com>
diff --git a/common/spl/spl_mmc.c b/common/spl/spl_mmc.c
index 0ab85d2..02ad32a 100644
--- a/common/spl/spl_mmc.c
+++ b/common/spl/spl_mmc.c
@@ -396,7 +396,7 @@
 #if !CONFIG_IS_ENABLED(BLK)
 	block_dev = &mmc->block_dev;
 #else
-	block_dev = dev_get_uclass_plat(mmc->dev);
+	block_dev = mmc_get_blk_desc(mmc);
 #endif
 	return block_dev->devnum;
 }