thermal: db8500_cpufreq_cooling: remove usage of IS_ERR_OR_NULL()

This patch changes the driver to avoid the usage of IS_ERR_OR_NULL()
macro. This macro can lead to dangerous results, like returning
success (0) during a failure scenario (NULL pointer handling).

The case present in this driver can be translated to a simple
check for IS_ERR(), as the cpufreq_cooling_register() returns
either a valid pointer or an ERR_PTR().

Cc: Zhang Rui <rui.zhang@intel.com>
Cc: Russell King <rmk@arm.linux.org.uk>
Cc: Grant Likely <grant.likely@secretlab.ca>
Cc: Rob Herring <rob.herring@calxeda.com>
Cc: Hongbo Zhang <hongbo.zhang@stericsson.com>
Cc: linux-pm@vger.kernel.org
Cc: linux-kernel@vger.kernel.org
Cc: devicetree-discuss@lists.ozlabs.org
Signed-off-by: Eduardo Valentin <eduardo.valentin@ti.com>
Acked-by: Fabio Baltieri <fabio.baltieri@linaro.org>
Signed-off-by: Zhang Rui <rui.zhang@intel.com>