iommu/fsl_pamu: Remove unnecessary null test before kfree

Fix checkpatch warning:
WARNING: kfree(NULL) is safe this check is probably not required

Cc: Joerg Roedel <joro@8bytes.org>
Cc: Grant Likely <grant.likely@linaro.org>
Cc: iommu@lists.linux-foundation.org
Cc: devicetree@vger.kernel.org
Signed-off-by: Fabian Frederick <fabf@skynet.be>
Signed-off-by: Joerg Roedel <jroedel@suse.de>