thermal/core: Delete device under thermal device zone lock

Thermal device attributes may still be opened after unregistering
the thermal zone and deleting the thermal device.

Currently there is no protection against accessing thermal device
operations after unregistering a thermal zone. To enable adding
such protection, protect the device delete operation with the
thermal zone device mutex. This requires splitting the call to
device_unregister() into its components, device_del() and put_device().
Only the first call can be executed under mutex protection, since
put_device() may result in releasing the thermal zone device memory.

Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>