omap iommu: fix incorrect address for largepage 1st entry

There was a potential risk that the following "memset" could override
its range if a given address was not the 1st entry of a
largepage. This is not the case for "iovmm".

Reported-by: Hari Nagalla <hnagalla@ti.com>
Signed-off-by: Hiroshi DOYU <Hiroshi.DOYU@nokia.com>
Signed-off-by: Hari Nagalla <hnagalla@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>