net: mvpp2: cls: Make the flow definitions const

The cls_flow table represent the overall configuration of the
classifier, used to match the different traffic classes in the Parsing
and Classification engines.

This configuration is static, and applies to all PPv2 instances, we must
therefore keep it const so that no modifications of this table are
performed at runtime.

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