cma: assign to correct variable in __iommu_alloc_buffer

When we moved to pfns in [9740ba42 "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.

Change-Id: I8fd920dff19a21ad441a35c0f772f947bfb756d9
Signed-off-by: Mitchel Humpherys <mitchelh@codeaurora.org>