iommu/tegra-smmu: Fix return value check in tegra_smmu_group_get()

In case of error, the function iommu_group_alloc() returns ERR_PTR() and
never returns NULL. The NULL test in the return value check should be
replaced with IS_ERR().

Fixes: 7f4c9176 ("iommu/tegra: Allow devices to be grouped")
Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Acked-by: Alex Williamson <alex.williamson@redhat.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>