iommu/omap: Fix compilation warnings

A recent patch introduced a new compiler warning because
two functions with non-void return type have no return
statement in omap-iommu.h for CONFIG_OMAP_IOMMU=n.

Fix this by adding return statements to these functions.

Fixes: d9c4d8a6 ('iommu/omap: introduce new API for runtime suspend/resume control')
Signed-off-by: Joerg Roedel <jroedel@suse.de>