xhci: replace 'xhci->cmd_ring->dequeue' with 'trb' in stop_ep cmd handler

This patch replaces 'xhci->cmd_ring->dequeue' with 'trb', the address of
the command TRB, since it is available to reduce line length.

Signed-off-by: Xenia Ragiadakou <burzalodowa@gmail.com>
Acked-by: Sarah Sharp <sarah.a.sharp@linux.intel.com>
Signed-off-by: Sarah Sharp <sarah.a.sharp@linux.intel.com>