iommu/amd: fix error return code in early_amd_iommu_init()

Fix to return -ENOMEM int the memory alloc error handling
case instead of 0, as done elsewhere in this function.

Signed-off-by: Wei Yongjun <yongjun_wei@trendmicro.com.cn>
Signed-off-by: Joerg Roedel <joro@8bytes.org>