dwc_otg: Call usb_hcd_unlink_urb_from_ep with lock held in completion tasklet

This fixes a regression introduced in c4564d4a1a0a9b10d4419e48239f5d99e88d2667,
usb_hcd_unlink_urb_from_ep must be called with the lock held.  Not doing so
resulted in corruption of the list.