net: mvpp2: make sure we don't spread load on disabled CPUs

When filling the RSS table, we have to make sure that the rx queue is
attached to an online CPU.

This patch is not a full support for cpu_hotplug, but rather a way to
make sure that we don't break network on system booted with the maxcpus
parameter.

Signed-off-by: Maxime Chevallier <maxime.chevallier@bootlin.com>
Signed-off-by: David S. Miller <davem@davemloft.net>