hwmon: (coretemp) Initialize tmin

ttarget is initialized when the driver is loaded, but tmin is not.
As a result, tempX_max_hyst attributes read 0. Fix this.

Also use THERM_*_THRESHOLD* constants in these initializations instead
of hard-coding the constants.

Signed-off-by: Jean Delvare <khali@linux-fr.org>
Cc: "R, Durgadoss" <durgadoss.r@intel.com>
Cc: Guenter Roeck <guenter.roeck@ericsson.com>
Cc: Fenghua Yu <fenghua.yu@intel.com>
Signed-off-by: Guenter Roeck <guenter.roeck@ericsson.com>