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/usb.c b/cmd/usb.c
index c7b642c..5f3b06a 100644
--- a/cmd/usb.c
+++ b/cmd/usb.c
@@ -625,7 +625,7 @@
 	int i;
 	extern char usb_started;
 #ifdef CONFIG_USB_STORAGE
-	block_dev_desc_t *stor_dev;
+	struct blk_desc *stor_dev;
 #endif
 
 	if (argc < 2)