xhci: Don't warn on empty ring for suspended devices.

When a device attached to the roothub is suspended, the endpoint rings
are stopped.  The host may generate a completion event with the
completion code set to 'Stopped' or 'Stopped Invalid' when the ring is
halted.  The current xHCI code prints a warning in that case, which can
be really annoying if the USB device is coming into and out of suspend.

Remove the unnecessary warning.

Signed-off-by: Sarah Sharp <sarah.a.sharp@linux.intel.com>
Tested-by: Stephen Hemminger <stephen@networkplumber.org>