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/rockusb.c b/cmd/rockusb.c
index 8206643..e0c1480 100644
--- a/cmd/rockusb.c
+++ b/cmd/rockusb.c
@@ -33,7 +33,7 @@
 	dev_index = simple_strtoul(devnum, NULL, 0);
 	rockusb_dev_init(devtype, dev_index);
 
-	ret = board_usb_init(controller_index, USB_INIT_DEVICE);
+	ret = usb_gadget_initialize(controller_index);
 	if (ret) {
 		printf("USB init failed: %d\n", ret);
 		return CMD_RET_FAILURE;
@@ -62,7 +62,7 @@
 exit:
 	g_dnl_unregister();
 	g_dnl_clear_detach();
-	board_usb_cleanup(controller_index, USB_INIT_DEVICE);
+	usb_gadget_release(controller_index);
 
 	return ret;
 }