ofw: Fix inverted bcmp in ofw_bus_node_status_okay

Otherwise this matches any two-character status except for ok.

Fixes: e5e94d2de987 ("Expand OpenFirmware API with ofw_bus_node_status_okay method")
MFC after: 1 week

(cherry picked from commit e1060f6dfd80b34cab6d439bf7420ad686ddc8f1)