thermal: devfreq: Simplify expression

There is no need to check for IS_ERR() as we are looking for a very
particular error value here. Drop the first check.

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Zhang Rui <rui.zhang@intel.com>