thermal/core: Introduce locked version of thermal_zone_device_update

In thermal_zone_device_set_mode(), the thermal zone mutex is released only
to be reacquired in the subsequent call to thermal_zone_device_update().

Introduce __thermal_zone_device_update(), which is similar to
thermal_zone_device_update() but has to be called with the thermal device
mutex held. Call the new function from thermal_zone_device_set_mode()
to avoid the extra thermal device mutex release/acquire sequence in that
function.

With the new function in place, re-implement thermal_zone_device_update()
as wrapper around __thermal_zone_device_update() to acquire and release
the thermal device mutex.

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