powerpc/pseries: Fix IBM_ARCH_VEC_NRCORES_OFFSET since POWER8NVL was added

The recent commit 7cc85103 ("powerpc/pseries: Add POWER8NVL support
to ibm,client-architecture-support call") added a new PVR mask & value
to the start of the ibm_architecture_vec[] array.

However it missed the fact that further down in the array, we hard code
the offset of one of the fields, and then at boot use that value to
patch the value in the array. This means every update to the array must
also update the #define, ugh.

This means that on pseries machines we will misreport to firmware the
number of cores we support, by a factor of threads_per_core.

Fix it for now by updating the #define.

Fixes: 7cc85103 ("powerpc/pseries: Add POWER8NVL support to ibm,client-architecture-support call")
Cc: stable@vger.kernel.org # v4.0+
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>