Darwin: Avoid retain cycle with MTRDevice (project-chip#32767)

Use the provided reference instead of capturing self.

Author: ksperling-apple

src/darwin/Framework/CHIP/MTRDevice.mm

@@ -1695,7 +1695,7 @@ - (void)_invokeCommandWithEndpointID:(NSNumber *)endpointID
         *isDuplicate = NO;
         *stop = YES;
     }];
-    [workItem setReadyHandler:^(MTRDevice * device, NSInteger retryCount, MTRAsyncWorkCompletionBlock workCompletion) {
+    [workItem setReadyHandler:^(MTRDevice * self, NSInteger retryCount, MTRAsyncWorkCompletionBlock workCompletion) {
         auto workDone = ^(NSArray<NSDictionary<NSString *, id> *> * _Nullable values, NSError * _Nullable error) {
             dispatch_async(queue, ^{
                 completion(values, error);