cpufreq: scpi: fix static checker warning cdev isn't an ERR_PTR

Commit 343a8d17 (cpufreq: scpi: remove arm_big_little dependency)
leads to the following static checker warning:

	drivers/cpufreq/scpi-cpufreq.c:203 scpi_cpufreq_ready()
	warn: 'cdev' isn't an ERR_PTR

of_cpufreq_cooling_register() returns NULL on error. This patch removes
the incorrect IS_ERR check on the returned pointer.

Fixes: 343a8d17 (cpufreq: scpi: remove arm_big_little dependency)
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>
Acked-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>