powerpc/powernv/ioda: Finish removing explicit max window size check

9003a249 removed checn from the DMA window pages allocator, however
the VFIO driver tests limits before doing so by calling
the get_table_size hook which was left behind; this fixes it.

Fixes: 9003a249 "powerpc/powernv/ioda: Remove explicit max window size check"
Signed-off-by: Alexey Kardashevskiy <aik@ozlabs.ru>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>