vfio/type1: Fix error return code in vfio_iommu_type1_attach_group()

Fix to return a negative error code from the error handling
case instead of 0, as done elsewhere in this function.

Fixes: 5d704992 ("vfio/type1: Allow transparent MSI IOVA allocation")
Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Reviewed-by: Eric Auger <eric.auger@redhat.com>
Acked-by: Alex Williamson <alex.williamson@redhat.com>
Signed-off-by: Joerg Roedel <jroedel@suse.de>