net: mvpp2: handle cases where more CPUs are available than s/w threads

The Marvell PPv2 network controller has 9 internal threads. The driver
works fine when there are less CPUs available than threads. This isn't
true if more CPUs are available. As this is a valid use case, handle
this particular case.

Signed-off-by: Antoine Tenart <antoine.tenart@bootlin.com>
Signed-off-by: David S. Miller <davem@davemloft.net>