iommu: arm-smmu: Fix list iteration error

After using list_for_each_entry, the iterator variable is set to the
head of the list even if the list is empty. Check for this and
return NULL in this case.

Change-Id: Ibd931f2353dee705b6be857e53854f1299219016
Signed-off-by: Patrick Daly <pdaly@codeaurora.org>