dm: block: Adjust device calls to go through helpers function

To ease conversion to driver model, add helper functions which deal with
calling each block device method. With driver model we can reimplement these
functions with the same arguments.

Use inline functions to avoid increasing code size on some boards.

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 53fd6ad..9ed5dc6 100644
--- a/cmd/usb.c
+++ b/cmd/usb.c
@@ -759,8 +759,7 @@
 			printf("\nUSB read: device %d block # %ld, count %ld"
 				" ... ", usb_stor_curr_dev, blk, cnt);
 			stor_dev = usb_stor_get_dev(usb_stor_curr_dev);
-			n = stor_dev->block_read(stor_dev, blk, cnt,
-						 (ulong *)addr);
+			n = blk_dread(stor_dev, blk, cnt, (ulong *)addr);
 			printf("%ld blocks read: %s\n", n,
 				(n == cnt) ? "OK" : "ERROR");
 			if (n == cnt)
@@ -781,8 +780,7 @@
 			printf("\nUSB write: device %d block # %ld, count %ld"
 				" ... ", usb_stor_curr_dev, blk, cnt);
 			stor_dev = usb_stor_get_dev(usb_stor_curr_dev);
-			n = stor_dev->block_write(stor_dev, blk, cnt,
-						(ulong *)addr);
+			n = blk_dwrite(stor_dev, blk, cnt, (ulong *)addr);
 			printf("%ld blocks write: %s\n", n,
 				(n == cnt) ? "OK" : "ERROR");
 			if (n == cnt)