iommu/exynos: Use generic group callback

Since iommu_group_get_for_dev() already tries iommu_group_get() and will
not call ops->device_group if the group is already non-NULL, the check
in get_device_iommu_group() is always redundant and it reduces to a
duplicate of the generic version; let's just use that one instead.

Signed-off-by: Robin Murphy <robin.murphy@arm.com>
Tested-by: Marek Szyprowski <m.szyprowski@samsung.com>
Signed-off-by: Joerg Roedel <jroedel@suse.de>