usb: host: xhci: check for a valid ring when unmapping bounce buffer

This way we can remove checks for valid ring from call sites of
xhci_unmap_td_bounce_buffer()

Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>
Signed-off-by: Mathias Nyman <mathias.nyman@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>