thermal: core: Restore behavior regarding invalid trip points

Commit 7c3d5c20 ("thermal/core: Add a generic thermal_zone_get_trip()
function") stopped marking trip points with a zero temperature as
disabled, behavior that was originally introduced in commit 81ad4276
("Thermal: Ignore invalid trip points").

When using the mlxsw driver we see that when such trip points are not
disabled, the thermal subsystem repeatedly tries to set the state of the
associated cooling devices to the maximum state.

Address this by restoring the original behavior and mark trip points
with a zero temperature as disabled.

Fixes: 7c3d5c20 ("thermal/core: Add a generic thermal_zone_get_trip() function")
Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>