platform/x86: mlx-platform: Fix tachometer registers

Shift by one the registers for tachometers (7 - 12).

This fix is relevant for the same new systems MQMB7, MSN37, MSN34,
which are about to be released to the customers.
At the moment, none of them is at customers sites. The customers will
not suffer from this change.
This fix is necessary, because register used before for tachometer 7
has been than reserved for the second PWM for newer systems, which are
not supported yet in mlx-platform driver. So registers of tachometers
7-12 have been shifted by one.

Fixes: 0378123c ("platform/x86: mlx-platform: Add mlxreg-fan platform driver activation")
Signed-off-by: Vadim Pasternak <vadimp@mellanox.com>
Signed-off-by: Darren Hart (VMware) <dvhart@infradead.org>