xhci(4): Always add and evaluate the slot context.

Because the maximum number of endpoint contexts is stored there.

Tested by: ehaupt@
PR: 262882
Approved by: re (gjb, early MFC)
Sponsored by: NVIDIA Networking

(cherry picked from commit 09dd1adfa4c9bb1b49f4ef5524a308732883e132)
(cherry picked from commit 6d8c6b24ee0a0416204356a98e4e7606489894c5)