[release-v2.0] jsonrpc: Allow mixaccount RPC to actually mix.

This RPC has been broken since dcrwallet was converted to peer-to-peer
mixing in bb04b75.

Backport of f5ebc3c.

internal/rpc/jsonrpc/methods.go

@@ -5641,7 +5641,7 @@ func (s *Server) mixOutput(ctx context.Context, icmd any) (any, error) {
 }
 
 func (s *Server) mixAccount(ctx context.Context, icmd any) (any, error) {
-	if s.cfg.Mixing {
+	if !s.cfg.Mixing {
 		return nil, errors.E("Mixing is not configured")
 	}
 	w, ok := s.walletLoader.LoadedWallet()