mlxsw: core: Zero payload buffers for couple of registers

We recently discovered a bug in the firmware in which a field's length in
one of the registers was incorrectly set. This caused the firmware to
access garbage data that wasn't initialized by the driver and therefore
emit error messages.

While the bug is already fixed and the driver usually zeros the buffers
passed to the firmware, there are a handful of cases where this isn't
done. Zero the buffer in these cases and prevent similar bugs from
recurring, as they tend to be hard to debug.

Fixes: 52581961

 ("mlxsw: core: Implement fan control using hwmon")
Signed-off-by: Elad Raz <eladr@mellanox.com>
Reviewed-by: Jiri Pirko <jiri@mellanox.com>
Reviewed-by: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>