iommu: dma-mapping-fast: Remove unused #define

There's a stray #define left over from initial development of the fast
map feature.  Rip it out.

CRs-Fixed: 1035430
Change-Id: Iae0649dc3fe878f948ad872803317eb4bf7ec1c6
Signed-off-by: Mitchel Humpherys <mitchelh@codeaurora.org>
Signed-off-by: Patrick Daly <pdaly@codeaurora.org>