powerpc/powernv: Fix it_ops::get() callback to return in cpu endian

The iommu_table_ops callbacks are declared CPU endian as they take and
return "unsigned long"; underlying hardware tables are big-endian.

However get() was missing be64_to_cpu(), this adds the missing conversion.

The only caller of this is crash dump at arch/powerpc/kernel/iommu.c,
iommu_table_clear() which only compares TCE to zero so this change
should not cause behavioral change.

Signed-off-by: Alexey Kardashevskiy <aik@ozlabs.ru>
Reviewed-by: David Gibson <david@gibson.dropbear.id.au>
Acked-by: Gavin Shan <gwshan@linux.vnet.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>