hwmon: (pmbus) Fix low limit temperature alarms

Temperature alarms are detected by checking the alarm bit and comparing
temperature limits against the current temperature. For low limits, this
comparison needs to be reversed (temp < limit instead of temp > limit).
This was not taken into account, resulting in wrong alarms if a temperature
fell below a low limit.

Fix by adding a low limit flag in the limit data structure. When creating the
sensor entry, the order of registers to compare is now reversed for low limits.

Signed-off-by: Guenter Roeck <guenter.roeck@ericsson.com>
Acked-by: Jean Delvare <khali@linux-fr.org>
Cc: stable@kernel.org # 3.0+