vfio/spapr_tce: Fix incorrect tce_iommu_group memory free

The @tcegrp variable is used in 1) a loop over attached groups
2) it stores a pointer to a newly allocated tce_iommu_group if 1) found
nothing. However the error handler does not distinguish how we got there
and incorrectly releases memory for a found+incompatible group.

This fixes it by adding another error handling case.

Fixes: 0bd97167

 ("powerpc/powernv/npu: Add compound IOMMU groups")
Signed-off-by: Alexey Kardashevskiy <aik@ozlabs.ru>
Reviewed-by: Paul Mackerras <paulus@ozlabs.org>
Signed-off-by: Alex Williamson <alex.williamson@redhat.com>