powerpc/xics/opal: Fix processor numbers in OPAL ICP

When using the OPAL ICP backend we incorrectly pass Linux CPU numbers
rather than HW CPU numbers to OPAL.

Fixes: d7436188

 ("powerpc/xics: Add ICP OPAL backend")
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>