xhci: For streams the dequeue ptr must be read from the stream ctx

This fixes TR dequeue validation failing on Intel XHCI controllers with the
following warning:

Mismatch between completed Set TR Deq Ptr command & xHCI internal state.

Interestingly enough reading the deq ptr from the ep ctx after a
TR Deq Ptr command does work on a Nec XHCI controller, it seems the Nec
writes the ptr to both the ep and stream contexts when streams are used.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Sarah Sharp <sarah.a.sharp@linux.intel.com>