global: Convert simple_strtoul() with decimal to dectoul()

It is a pain to have to specify the value 10 in each call. Add a new
dectoul() function and update the code to use it.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/cmd/usb.c b/cmd/usb.c
index b9ec29a..3d87376 100644
--- a/cmd/usb.c
+++ b/cmd/usb.c
@@ -690,7 +690,7 @@
 			 * have multiple controllers and the device numbering
 			 * starts at 1 on each bus.
 			 */
-			i = simple_strtoul(argv[2], NULL, 10);
+			i = dectoul(argv[2], NULL);
 			printf("config for device %d\n", i);
 			udev = usb_find_device(i);
 			if (udev == NULL) {
@@ -706,13 +706,13 @@
 	if (strncmp(argv[1], "test", 4) == 0) {
 		if (argc < 5)
 			return CMD_RET_USAGE;
-		i = simple_strtoul(argv[2], NULL, 10);
+		i = dectoul(argv[2], NULL);
 		udev = usb_find_device(i);
 		if (udev == NULL) {
 			printf("Device %d does not exist.\n", i);
 			return 1;
 		}
-		i = simple_strtoul(argv[3], NULL, 10);
+		i = dectoul(argv[3], NULL);
 		return usb_test(udev, i, argv[4]);
 	}
 #ifdef CONFIG_USB_STORAGE