powerpc/powernv/ioda: Fix endianness when reading TCEs

The iommu_table_ops::exchange() callback writes new TCE to the table and
returns old value and permission mask. The old TCE value is correctly
converted from BE to CPU endian; however permission mask was calculated
from BE value and therefore always returned DMA_NONE which could cause
memory leak on LE systems using VFIO SPAPR TCE IOMMU v1 driver.

This fixes pnv_tce_xchg() to have @oldtce a CPU endian.

Fixes: 05c6cfb9

 ("powerpc/iommu/powernv: Release replaced TCE")
Signed-off-by: Alexey Kardashevskiy <aik@ozlabs.ru>
Reviewed-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>