wifi: libertas: use USB anchors for tracking in-flight URBs

The libertas driver currently handles URB lifecycles manually, which
makes it non-trivial to check if specific URBs are pending or not. Add
anchors for TX/RX URBs, and use those to track in-flight requests.

Signed-off-by: Heitor Alves de Siqueira <halves@igalia.com>
Link: https://patch.msgid.link/20260313-libertas-usb-anchors-v1-1-915afbe988d7@igalia.com
Signed-off-by: Johannes Berg <johannes.berg@intel.com>