pnv/xive2: Permit valid writes to VC/PC Flush Control registers

pnv/xive2: Permit valid writes to VC/PC Flush Control registers

Writes to the Flush Control registers were logged as invalid
when they are allowed. Clearing the unsupported want_cache_disable
feature is supported, so don't log an error in that case.

Signed-off-by: Michael Kowal <kowal@linux.ibm.com>
Reviewed-by: Nicholas Piggin <npiggin@gmail.com>
Reviewed-by: Glenn Miles <milesg@linux.ibm.com>
Reviewed-by: Michael Kowal <kowal@linux.ibm.com>
Reviewed-by: Caleb Schlossin <calebs@linux.ibm.com>
Tested-by: Gautam Menghani <gautam@linux.ibm.com>
Link: https://lore.kernel.org/qemu-devel/20250512031100.439842-21-npiggin@gmail.com
Signed-off-by: Cédric Le Goater <clg@redhat.com>