xhci: get isochronous ring directly from endpoint structure

isochronous endpoints do not support streams, meaning that
there is only one ring per endpoint.

Avoid double-fetching the transfer event DMA to get the
ring. Also makes passing the event to skip_isoc_td() uncecessary.

Signed-off-by: Mathias Nyman <mathias.nyman@linux.intel.com>
Link: https://lore.kernel.org/r/20210129130044.206855-3-mathias.nyman@linux.intel.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>