usb: xhci: Fix invalid pointer dereference in Etron workaround

This check is performed before prepare_transfer() and prepare_ring(), so
enqueue can already point at the final link TRB of a segment. And indeed
it will, some 0.4% of times this code is called.

Then enqueue + 1 is an invalid pointer. It will crash the kernel right
away or load some junk which may look like a link TRB and cause the real
link TRB to be replaced with a NOOP. This wouldn't end well.

Use a functionally equivalent test which doesn't dereference the pointer
and always gives correct result.

Something has crashed my machine twice in recent days while playing with
an Etron HC, and a control transfer stress test ran for confirmation has
just crashed it again. The same test passes with this patch applied.

Fixes: 5e1c67abc930 ("xhci: Fix control transfer error on Etron xHCI host")
Cc: [e-mail]
Signed-off-by: Michal Pecio <[e-mail]>
Signed-off-by: Mathias Nyman <[e-mail]>
Reviewed-by: Kuangyi Chiang <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Greg Kroah-Hartman <[e-mail]>

drivers/usb/host/xhci-ring.c

···
 		 * enqueue a No Op TRB, this can prevent the Setup and Data Stage
 		 * TRB to be breaked by the Link TRB.
 		 */
-		if (trb_is_link(ep_ring->enqueue + 1)) {
+		if (last_trb_on_seg(ep_ring->enq_seg, ep_ring->enqueue + 1)) {
 			field = TRB_TYPE(TRB_TR_NOOP) | ep_ring->cycle_state;
 			queue_trb(xhci, ep_ring, false, 0, 0,
 					TRB_INTR_TARGET(0), field);