omap: iommu: PREFETCH_IOTLB cleanup

Use PREFETCH_IOTLB to control the content of the called function,
instead of inlining it in the code.

This improves readability of the code, and also prevents an "unused
function" warning to show up when PREFETCH_IOTLB isn't set.

Signed-off-by: Ohad Ben-Cohen <ohad@wizery.com>
Acked-by: Hiroshi DOYU <Hiroshi.DOYU@nokia.com>
Acked-by: Tony Lindgren <tony@atomide.com>
Signed-off-by: Joerg Roedel <joerg.roedel@amd.com>