thermal: step_wise: fix: Prevent from binary overflow when trend is dropping

It turns out that some boards can have instance->lower greater than 0 and
when thermal trend is dropping it results with next_target equal to -1.

Since the next_target is defined as unsigned long it is interpreted as
0xFFFFFFFF and larger than instance->upper.
As a result the next_target is set to instance->upper which ramps up to
maximal cooling device target when the temperature is steadily decreasing.

Signed-off-by: Lukasz Majewski <l.majewski@samsung.com>
Signed-off-by: Zhang Rui <rui.zhang@intel.com>