iommu/vt-d: Use right Kconfig option name

commit ba61c3da upstream.

The CONFIG_ prefix should be added in the code.

Fixes: 04618252

 ("iommu/vt-d: Add Kconfig option to enable/disable scalable mode")
Reported-and-tested-by: Kumar, Sanjay K <sanjay.k.kumar@intel.com>
Signed-off-by: Lu Baolu <baolu.lu@linux.intel.com>
Cc: Ashok Raj <ashok.raj@intel.com>
Link: https://lore.kernel.org/r/20200501072427.14265-1-baolu.lu@linux.intel.com
Signed-off-by: Joerg Roedel <jroedel@suse.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>