fix comment typo in pci-dma.c

 * This variable becomes 1 if iommu=pt is passed on the kernel command line.

 * If this variable is 1, IOMMU implementations do no DMA translation for

 * devices and allow every device to access to whole physical memory. This is

 * useful if a user want to use an IOMMU only for KVM device assignment to

 * useful if a user wants to use an IOMMU only for KVM device assignment to

 * guests and not for driver dma translation.

 */

int iommu_pass_through __read_mostly;