powerpc/powernv/ioda: Remove explicit max window size check

DMA windows can only have a size of power of two on IODA2 hardware and
using memory_hotplug_max() to determine the upper limit won't work
correcly if it returns not power of two value.

This removes the check as the platform code does this check in
pnv_pci_ioda2_setup_default_config() anyway; the other client is VFIO
and that thing checks against locked_vm limit which prevents the userspace
from locking too much memory.

It is expected to impact DPDK on machines with non-power-of-two RAM size,
mostly. KVM guests are less likely to be affected as usually guests get
less than half of hosts RAM.

Signed-off-by: Alexey Kardashevskiy <aik@ozlabs.ru>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>