Fix an undefined variable that was accidentally not causing an error.

The code had references to both intr_offset and intr_parent variable names
as referring to the parent interrupt node.  The intr_parent variable
wasn't actually defined anywhere, but the only references to it were as
an argument to a macro that didn't use that argument in expansion, so
the undefined variable accidentally didn't cause an error.

The intr_parent name makes more sense in context, so change all occurrances
of intr_offset to intr_parent.