usb: gadget: Do not call board_usb_xxx() directly in USB gadget drivers

Add 2 functions to wrap the calls to board_usb_init() and
board_usb_cleanup().
This is a preparatory work for DM support for UDC drivers (DM_USB_GADGET).

Signed-off-by: Jean-Jacques Hiblot <jjhiblot@ti.com>
Reviewed-by: Lukasz Majewski <lukma@denx.de>
diff --git a/cmd/thordown.c b/cmd/thordown.c
index 2615ada..ce3660d 100644
--- a/cmd/thordown.c
+++ b/cmd/thordown.c
@@ -30,7 +30,7 @@
 		goto done;
 
 	int controller_index = simple_strtoul(usb_controller, NULL, 0);
-	ret = board_usb_init(controller_index, USB_INIT_DEVICE);
+	ret = usb_gadget_initialize(controller_index);
 	if (ret) {
 		pr_err("USB init failed: %d\n", ret);
 		ret = CMD_RET_FAILURE;
@@ -55,7 +55,7 @@
 
 exit:
 	g_dnl_unregister();
-	board_usb_cleanup(controller_index, USB_INIT_DEVICE);
+	usb_gadget_release(controller_index);
 done:
 	dfu_free_entities();