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: 14295a150050 ("usb: xhci-mtk: support to build xhci-mtk-hcd.ko")
Reported-by: Eddie Hung <[e-mail]>
Signed-off-by: Chunfeng Yun <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Greg Kroah-Hartman <[e-mail]>

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

···
 	if (ret)
 		return ret;
 
-	drop_ep_quirk(hcd, udev, ep);
+	if (ep->hcpriv)
+		drop_ep_quirk(hcd, udev, ep);
+
 	return 0;
 }