iommu/ipmmu-vmsa: Fix return value check in ipmmu_find_group_dma()

In case of error, the function iommu_group_get() returns NULL pointer
not ERR_PTR(). The IS_ERR() test in the return value check should be
replaced with NULL test.

Fixes: 3ae47292 ("iommu/ipmmu-vmsa: Add new IOMMU_DOMAIN_DMA ops")
Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Signed-off-by: Alex Williamson <alex.williamson@redhat.com>