iommu: Remove extra NULL check when call strtobool()

strtobool() does check for NULL parameter already. No need to repeat.

While here, switch to kstrtobool() and unshadow actual error code
(which is still -EINVAL).

No functional change intended.

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