iommu/fsl: Fix the error condition during iommu group

Earlier PTR_ERR was being returned even if group was set to null.
Now, we explicitly set an ERR_PTR value in case the group  pointer is
NULL.

Signed-off-by: Varun Sethi <Varun.Sethi@freescale.com>
Signed-off-by: Joerg Roedel <jroedel@suse.de>