dm: Drop the block_dev_desc_t typedef

Use 'struct' instead of a typdef. Also since 'struct block_dev_desc' is long
and causes 80-column violations, rename it to struct blk_desc.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Tested-by: Stephen Warren <swarren@nvidia.com>
diff --git a/cmd/disk.c b/cmd/disk.c
index 3025225..c53c2a2 100644
--- a/cmd/disk.c
+++ b/cmd/disk.c
@@ -20,7 +20,7 @@
 #if defined(CONFIG_IMAGE_FORMAT_LEGACY)
 	image_header_t *hdr;
 #endif
-	block_dev_desc_t *dev_desc;
+	struct blk_desc *dev_desc;
 
 #if defined(CONFIG_FIT)
 	const void *fit_hdr = NULL;