hwmon: (k10temp) Only apply temperature offset if result is positive

A user reports a really bad temperature on Ryzen 1950X.

k10temp-pci-00cb
Adapter: PCI adapter
temp1: +4294948.3°C (high = +70.0°C)

This will happen if the temperature reported by the chip is lower than
the offset temperature. This has been seen in the field if "Sense MI Skew"
and/or "Sense MI Offset" BIOS parameters were set to unexpected values.
Let's report a temperature of 0 degrees C in that case.

Fixes: 1b50b776

 ("hwmon: (k10temp) Add support for temperature offsets")
Signed-off-by: Guenter Roeck <linux@roeck-us.net>