ACPI: NUMA: Optimize the check for the availability of node values

The first_unset_node() function returns the first unused node in
nodes_found_map. If all nodes are in use, the function returns
MAX_NUMNODES.

Use this return value to determine whether there are any available node
values in nodes_found_map, eliminating the need to use nodes_weight()
for this purpose.

Signed-off-by: Yuntao Wang <ytcoode@gmail.com>
[ rjw: Changelog edits ]
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>