omap iommu: move iommu_disable at fault to the above layer

The function prefix "omap2_iommu_" indicates that the prefixed
function belongs to "omap2_iommu_ops" to provide iommu basic
functionalities for the above layers. It's better to avoid the
prefixed function called in the same prefixed ones internally, like
nested here. Now "iommu_disable" is called just after fault_isr() in
the above layer. This is a little bit more sensible to keep the
consistency of module layers.

Signed-off-by: Hiroshi DOYU <Hiroshi.DOYU@nokia.com>