xhci: Kill streams URBs when the host dies.

If the host controller stops responding to commands, we need to kill all
the URBs that were queued to all endpoints.  The current code would only
kill URBs that had been queued to the endpoint rings.  ep->ring is set
to NULL if streams has been enabled for the endpoint, which means URBs
submitted with a non-zero stream_id would never get killed.  Fix this.

Signed-off-by: Sarah Sharp <sarah.a.sharp@linux.intel.com>