ppc/pnv: PNV I2C engines assigned incorrect XSCOM addresses

ppc/pnv: PNV I2C engines assigned incorrect XSCOM addresses

The PNV I2C engines for power9 and power10 were being assigned a base
XSCOM address that was off by one I2C engine's address range such
that engine 0 had engine 1's address and so on.  The xscom address
assignment was being based on the device tree engine numbering, which
starts at 1.  Rather than changing the device tree numbering to start
with 0, the addressing was changed to be based on the existing device
tree numbers minus one.

Fixes: 1ceda19c28a1 ("ppc/pnv: Connect PNV I2C controller to powernv10)
Signed-off-by: Glenn Miles <milesg@linux.vnet.ibm.com>
Signed-off-by: Cédric Le Goater <clg@kaod.org>