iommu/vt-d: Fix memory leak in dmar_insert_one_dev_info()

We are returning NULL if we are not able to attach the iommu
to the domain but while returning we missed freeing info.

Signed-off-by: Sudip Mukherjee <sudip@vectorindia.org>
Signed-off-by: Joerg Roedel <jroedel@suse.de>