x86/hwmon: register alternate sibling upon CPU removal

Just like pkgtemp registers another core of the same package when one
gets removed, coretemp should register another hyperthread (if
available) in that situation.

As pointed out in the patch fixing the respective code in pkgtemp, the
list protectng mutex must be dropped before calling
coretemp_device_add(), and due to the restructured loop (including an
explicit return) the "safe" variant of the list iterator isn't needed
anymore.

Signed-off-by: Jan Beulich <jbeulich@novell.com>
Cc: Rudolf Marek <r.marek@assembler.cz>
Signed-off-by: Guenter Roeck <guenter.roeck@ericsson.com>