hwmon: coretemp: update hotplug condition check

Fix two errors in hotplug.  One is for hotplug notifier.  The other is
unnecessary driver unregister.  Because even none of online cpus supports
coretemp, we can't assume new onlined cpu doesn't support it either.  If
related driver is unregistered there we have no chance to use coretemp
from then on.

Signed-off-by: Chen Gong <gong.chen@linux.intel.com>
Cc: Rudolf Marek <r.marek@assembler.cz>
Cc: Huaxu Wan <huaxu.wan@intel.com>
Cc: Jean Delvare <khali@linux-fr.org>
Cc: Guenter Roeck <guenter.roeck@ericsson.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>