nfp: flower: validate merge hint flows

Two flows can be merged if the second flow (after recirculation) matches
on bits that are either matched on or explicitly set by the first flow.
This means that if a packet hits flow 1 and recirculates then it is
guaranteed to hit flow 2.

Add a 'can_merge' function that determines if 2 sub_flows in a merge hint
can be validly merged to a single flow.

Signed-off-by: John Hurley <john.hurley@netronome.com>
Signed-off-by: Simon Horman <simon.horman@netronome.com>
Signed-off-by: David S. Miller <davem@davemloft.net>