net: mvpp2: use RSS only when using multi-queue mode

Since RSS only applies when we have per-cpu rx queues, it should only
be enabled when the driver is configured to make use of multi-queue
mode.

Signed-off-by: Yan Markman <ymarkman@marvell.com>
[Maxime: Commit message]
Signed-off-by: Maxime Chevallier <maxime.chevallier@bootlin.com>
Signed-off-by: David S. Miller <davem@davemloft.net>