x86/hwmon: fix module init for hotplug-but-no-device-found case

In commit 0dca94ba

 the call to
platform_driver_unregister() was made conditional upon !HOTPLUG_CPU,
but the return value from coretemp_init() was left to indicate an
error. This isn't correct, as the negative return value indicates to
the module loader that initialization failed, which isn't intended
here and results in dangling pointers.

Signed-off-by: Jan Beulich <jbeulich@novell.com>
Cc: Chen Gong <gong.chen@linux.intel.com>
Signed-off-by: Guenter Roeck <guenter.roeck@ericsson.com>