cma: assign to correct variable in __iommu_alloc_buffer

When we moved to pfns in [9740ba422e "cma: use pfn instead of pages for
argument passing"], there was one place where we should have been using
a page variable instead of a pfn. Fix this.

Signed-off-by: Mitchel Humpherys <mitchelh@codeaurora.org>
Signed-off-by: Pranav Vashi <neobuddy89@gmail.com>