iommu/s390: Use iommu_group_get_for_dev() in s390_iommu_add_device()

The iommu_group_get_for_dev() function also attaches the
device to its group, so this code doesn't need to be in the
iommu driver.

Further by using this function the driver can make use of
default domains in the future.

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