iommu: sprd: Avoid NULL deref in sprd_iommu_hw_en

In the Linux kernel, the following vulnerability has been resolved: iommu: sprd: Avoid NULL deref in sprd_iommu_hw_en In sprd_iommu_cleanup() before calling function sprd_iommu_hw_en() dom->sdev is equal to NULL, which leads to null dereference. Found by Linux Verification Center (linuxtesting.org) with SVACE.