net: mvpp2: cls: Start cls flow entries from beginning of table

The Classifier flow table has 512 entries, that contains lookups
commands executed consecutively for every flow. Since we have 21
different flows, we have to carefully manage the flow table use.

As of today, the start index of a lookup sequence is computed
directly based in the flow->id. There are 8 reserved flow ids, from
0-7, which don't have any corresponding sequence in the flow table. We
can therefore ignore them when computing the index, and make so that the
first non-reserved flow point to the very beginning of the flow table.

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