xhci: use completion event's slot id rather than dig it out of command

Since the slot id retrieved from the Reset Device TRB matches the slot id in
the command completion event, which is available, there is no need to determine
it again.
This patch removes the uneccessary reassignment to slot id and adds a WARN_ON
in case the two Slot ID fields differ (although according xhci spec rev1.0
they should not differ).

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