iommu: Return ERR_PTR() values from device_group call-backs

The generic device_group call-backs in iommu.c return NULL
in case of error. Since they are getting ERR_PTR values from
iommu_group_alloc(), just pass them up instead.

Reported-by: Gerald Schaefer <gerald.schaefer@de.ibm.com>
Signed-off-by: Joerg Roedel <jroedel@suse.de>