iommu/omap: Remove unnecessary error traces on alloc failures

Fix couple of checkpatch warnings of the type,
    "WARNING: Possible unnecessary 'out of memory' message"

Signed-off-by: Suman Anna <s-anna@ti.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Joerg Roedel <jroedel@suse.de>