expo: Use a simpler name for USB media

For USB devices we use the string 'usb_mass_storage' which is a bit
verbose. Use 'USB' instead.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/boot/bootflow_menu.c b/boot/bootflow_menu.c
index ab21ed8..7b44b64 100644
--- a/boot/bootflow_menu.c
+++ b/boot/bootflow_menu.c
@@ -84,7 +84,9 @@
 {
 	struct menu_priv *priv = exp->priv;
 	char str[2], *label, *key;
+	struct udevice *media;
 	struct scene *scn;
+	const char *name;
 	uint preview_id;
 	uint scene_id;
 	bool add_gap;
@@ -101,7 +103,14 @@
 	key = strdup(str);
 	if (!key)
 		return log_msg_ret("key", -ENOMEM);
-	label = strdup(dev_get_parent(bflow->dev)->name);
+
+	media = dev_get_parent(bflow->dev);
+	if (device_get_uclass_id(media) == UCLASS_MASS_STORAGE)
+		name = "usb";
+	else
+		name = media->name;
+	label = strdup(name);
+
 	if (!label) {
 		free(key);
 		return log_msg_ret("nam", -ENOMEM);