net: mvpp2: cls: Initialize lookup priorities for all entries in the flow

When classifying a packet pertaining to a given flow, the classifier
will issue multiple lookup commands until it finds one with the 'last'
bit set. It expects all prorities to be assign continuously (although
not necessarily in an ordered fashion) from 0 to the number of lookups.

We can initialize this once, and make sure unused lookups are given an
empty port map. This avoids having to maintain priorities and the
information of which lookup is the last.

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