powerpc/iommu/debug: fix ifnullfree.cocci warnings

arch/powerpc/kernel/iommu.c:76:2-16: WARNING: NULL check before some freeing functions is not needed.

 NULL check before some freeing functions is not needed.

 Based on checkpatch warning
 "kfree(NULL) is safe this check is probably not required"
 and kfreeaddr.cocci by Julia Lawall.

Generated by: scripts/coccinelle/free/ifnullfree.cocci

Fixes: 691602aa

 ("powerpc/iommu/debug: Add debugfs entries for IOMMU tables")
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: kernel test robot <lkp@intel.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20210318234441.GA63469@f8e20a472e81