thermal/core: Protect hwmon accesses to thermal operations with thermal zone mutex

In preparation to protecting access to thermal operations against thermal
zone device removal, protect hwmon accesses to thermal zone operations
with the thermal zone mutex. After acquiring the mutex, ensure that the
thermal zone device is registered before proceeding.

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