hwmon: (coretemp) Simplify sibling management

The coretemp driver provides a sysfs interface per physical core. If
hyperthreading is enabled and one of the siblings goes offline the sysfs
interface is removed and then immeditately created again for the
sibling. The only difference of them is the target cpu for the
rdmsr_on_cpu() in the sysfs show functions.

It's way simpler to keep a cpumask of cpus which are active in a package
and only remove the interface when the last sibling goes offline. Otherwise
just move the target cpu for the sysfs show functions to the still online
sibling.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>