iwlwifi: mvm: Populate trip table before registering thermal zone

The trip table in iwl_mvm_thermal_zone_register() is populated after
passing it to thermal_zone_device_register_with_trips(), so it may be
accessed (for instance, via sysfs) before it is ready.

To prevent that from happening, modify the function to populate the
trip table before calling thermal_zone_device_register_with_trips().

Also make the code use THERMAL_TEMP_INVALID as the "invalid temperature"
value which is also meaningful for the core.

Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Acked-by: Miri Korenblit <Miriam.rachel.korenblit@intel.com>