ppc/spapr-caps: Disallow setting workaround for spapr-cap-ibs

The spapr-cap cap-ibs can only have values broken or fixed as there is
no explicit workaround required. Currently setting the value workaround
for this cap will hit an assert if the guest makes the hcall
h_get_cpu_characteristics.

Report an error when attempting to apply the setting with a more helpful
error message.

Reported-by: Satheesh Rajendran <sathnaga@linux.vnet.ibm.com>
Signed-off-by: Suraj Jitindar Singh <sjitindarsingh@gmail.com>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>