devfreq_cooling: no need to check state with negative number

We could see that state is defined as unsigned type, so it
should never be less than zero. Let' remove this check.

Signed-off-by: Shawn Lin <shawn.lin@rock-chips.com>
Reviewed-by: Chanwoo Choi <cw00.choi@samsung.com>
Signed-off-by: Zhang Rui <rui.zhang@intel.com>