iommu/dmar: Remove redundant assignment of ret

There is no need to assign ret to 0 in some cases. Moreover it might
shadow some errors in the future.

Remove such assignments.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Joerg Roedel <jroedel@suse.de>