`setup_ndstats_sum` needs to operate on the stats plugin, not the hdf one

[coretl]

ophyd-async/src/ophyd_async/plan_stubs/_nd_attributes.py

Lines 52 to 57 in 9ea28ad

	def setup_ndstats_sum(detector: Device):
	hdf = getattr(detector, "hdf", None)
	assert isinstance(hdf, NDFileHDFIO), (
	f"Expected {detector.name} to have 'hdf' attribute that is an NDFilHDFIO, "
	f"got {hdf}"
	)