pseries: Clean up error handling in spapr_rtas_register()

pseries: Clean up error handling in spapr_rtas_register()

The errors detected in this function necessarily indicate bugs in the rest
of the qemu code, rather than an external or configuration problem.

So, a simple assert() is more appropriate than any more complex error
reporting.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Alexey Kardashevskiy <aik@ozlabs.ru>
Reviewed-by: Markus Armbruster <armbru@redhat.com>