iommu/omap: Remove conditional definition of dev_to_omap_iommu()
The dev_to_omap_iommu() is local to the OMAP IOMMU modules, and
need not be defined conditionally. The CONFIG_IOMMU_API dependency
check was added in the past to fix a compilation issue back when
the header resided in the arch/arm layers, and is no longer
needed.
While at this, fix the header against double inclusion as well.