usb: xhci-mtk: fix oops when unbind driver

The oops happens when unbind driver through sysfs as following,
because xhci_mtk_drop_ep() try to drop the endpoint of root hub
which is not added by xhci_add_endpoint() and the virtual device
is not allocated, in fact also needn't drop it, so should skip it.

Call trace:
 xhci_mtk_drop_ep+0x1b8/0x298
 usb_hcd_alloc_bandwidth+0x1d8/0x380
 usb_disable_device_endpoints+0x8c/0xe0
 usb_disable_device+0x128/0x168
 usb_disconnect+0xbc/0x2c8
 usb_remove_hcd+0xd8/0x210
 xhci_mtk_remove+0x98/0x108
 platform_remove+0x28/0x60
 device_release_driver_internal+0x110/0x1e8
 device_driver_detach+0x18/0x28
 unbind_store+0xd4/0x108
 drv_attr_store+0x24/0x38

Fixes: 14295a1500 ("usb: xhci-mtk: support to build xhci-mtk-hcd.ko")
Reported-by: Eddie Hung <eddie.hung@mediatek.com>
Signed-off-by: Chunfeng Yun <chunfeng.yun@mediatek.com>
Link: https://lore.kernel.org/r/1617179142-2681-2-git-send-email-chunfeng.yun@mediatek.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/usb/host/xhci-mtk-sch.c

@@ -872,6 +872,8 @@ int xhci_mtk_drop_ep(struct usb_hcd *hcd, struct usb_device *udev,
 	if (ret)
 		return ret;
 
-	drop_ep_quirk(hcd, udev, ep);
+	if (ep->hcpriv)
+		drop_ep_quirk(hcd, udev, ep);
+
 	return 0;
 }