mlxsw: core_hwmon: Query MTMP before writing to set only relevant fields

The MTMP register controls various temperature settings on a per-sensor
basis. Subsequent patches are going to alter some of these settings for
sensors found on port modules in response to certain events.

In order to prevent the current callers that write to MTMP from
overriding these settings, have them first query the register and then
change only the relevant register fields.

Signed-off-by: Amit Cohen <amcohen@nvidia.com>
Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>