Update amaranth/vendor/_xilinx.py

Co-authored-by: Catherine <whitequark@whitequark.org>

Author: daniestevez

amaranth/vendor/_xilinx.py

@@ -1215,7 +1215,7 @@ def get_async_ff_sync(self, async_ff_sync):
                               a_ASYNC_REG="TRUE")
                      for i in range(async_ff_sync._stages)]
         for i, f in enumerate(flops):
-            setattr(m.submodules, f"stage{i}", f)
+            m.submodules[f"stage{i}"] = f
         if self.toolchain == "Vivado":
             if async_ff_sync._max_input_delay is None:
                 # This attribute should be used with a constraint of the form