arm/arm64: trivial: another assert fix

Signed-off-by: Andrew Jones <drjones@redhat.com>
Reviewed-by: David Gibson <david@gibson.dropbear.id.au>
Reviewed-by: Thomas Huth <thuth@redhat.com>

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Unrelated to the powerpc initial drop, but found it while working
on that, so just posting it now. Anyway, it's a trivial fix.