POWER9 DD1 uses RTS - 28 for the RTS value but other revisions use
RTS - 31.  This makes this distinction for the different revisions
Signed-off-by: NAneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>
Acked-by: NMichael Neuling <mikey@neuling.org>
Signed-off-by: NMichael Ellerman <mpe@ellerman.id.au>