x86/hwmon: fix initialization of pkgtemp

Feature availability should also be checked in the hotplug code path.

Signed-off-by: Jan Beulich <jbeulich@novell.com>
Cc: Fenghua Yu <fenghua.yu@intel.com>
Signed-off-by: Guenter Roeck <guenter.roeck@ericsson.com>