mlxsw: core: Use different get_trend() callbacks for different thermal zones

The driver registers three different types of thermal zones: For the
ASIC itself, for port modules and for gearboxes.

Currently, all three types use the same get_trend() callback which does
not work correctly for the ASIC thermal zone. The callback assumes that
the device data is of type 'struct mlxsw_thermal_module', whereas for
the ASIC thermal zone 'struct mlxsw_thermal' is passed as device data.

Fix this by using one get_trend() callback for the ASIC thermal zone and
another for the other two types.

Fixes: 6f73862f

 ("mlxsw: core: Add the hottest thermal zone detection")
Signed-off-by: Vadim Pasternak <vadimp@mellanox.com>
Reviewed-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>