spapr: Assume msi_nonbroken

We conditionally adjust part of the guest device tree based on the
global msi_nonbroken flag.  However, the main machine type code
initializes msi_nonbroken to true and there's nothing that would set
it to false again.

So replace the test with an assert().

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Reviewed-by: Alexey Kardashevskiy <aik@ozlabs.ru>
