net: mvpp2: make sure we use single queue mode on PPv2.1

The PPv2 driver defines 2 "queue_modes" :
 - QDIST_SINGLE_MODE, where each port share one rx queue vector
   between all CPUs
 - QDIST_MULTI_MODE, where each port has one rx queue vector per CPU.

Multi queue mode isn't available on PPv2.1, make sure we fallback to
single mode when running on this revision.

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