Fixes variable usage in bigip device trust (#40975)

Variable used was incorrectly named

lib/ansible/modules/network/f5/bigip_device_trust.py

@@ -232,7 +232,7 @@ class ModuleManager(object):
         if self.want.password:
             return self.password
         if self.want.provider.get('password', None):
-            return self.provider.get('password')
+            return self.want.provider.get('password')
         if self.module.params.get('password', None):
             return self.module.params.get('password')