thermal: trip: Split thermal_zone_device_set_mode()

Pull a wrapper around thermal zone .change_mode() callback out of
thermal_zone_device_set_mode() because it will be used elsewhere
subsequently.

No intentional functional impact.

Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Link: https://patch.msgid.link/2206793.irdbgypaU6@rjwysocki.net