thermal/core: Check get_temp ops is present when registering a tz

Initially the check against the get_temp ops in the
thermal_zone_device_update() was put in there in order to catch
drivers not providing this method.

Instead of checking again and again the function if the ops exists in
the update function, let's do the check at registration time, so it is
checked one time and for all.

Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>