The box_ids only save the unit ID for the first die. If a unit, e.g., a
CXL unit, doesn't exist in the first die. The unit ID cannot be
retrieved.

The unit control RB tree also stores the unit ID information.
Retrieve the unit ID from the unit control RB tree
Signed-off-by: Kan Liang <kan.liang@linux.intel.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Tested-by: Yunying Sun <yunying.sun@intel.com>
Link: https://lore.kernel.org/r/20240614134631.1092359-4-kan.liang@linux.intel.com