Red Hat: CVE-2025-37813: kernel: usb: xhci: Fix invalid pointer dereference in Etron workaround

In the Linux kernel, the following vulnerability has been resolved:

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.