x86/hwmon: don't leak device attribute file from pkgtemp_probe() and pkgtemp_remove()

While apparently inherited from coretemp source, this particular error handling
cleanup and exit path wasn't copied properly (or perhaps got discarded
intermediately and not re-added properly later).

Signed-off-by: Jan Beulich <jbeulich@novell.com>
Cc: Fenghua Yu <fenghua.yu@intel.com>
Signed-off-by: Guenter Roeck <guenter.roeck@ericsson.com>
(added device file removal in pkgtemp_remove)