usb: xhci: remove xhci_dbg_ep_rings()

xhci_dbg_ep_rings() isn't used in xhci driver anymore. Remove
it to reduce the module binary size.

Signed-off-by: Lu Baolu <baolu.lu@linux.intel.com>
Signed-off-by: Mathias Nyman <mathias.nyman@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>