iommu/dmar: Remove redundant ' != 0' when check return code

Usual pattern when we check for return code, which might be negative
errno, is either (ret) or (!ret).

Remove extra ' != 0' from condition.

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