xhci: add variable 'cmd_trb' in handle_cmd_completion()

This patch adds a new variable 'cmd_trb' to hold the address of the
command TRB, that is associated with the command completion event,
and to replace repetitions of xhci->cmd_ring->dequeue into the code.

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>