nfp: extend flower add flow offload

Extends the flower flow add function by calculating which match
fields are present in the flower offload structure and allocating
the appropriate space to describe these.

Signed-off-by: Pieter Jansen van Vuuren <pieter.jansenvanvuuren@netronome.com>
Signed-off-by: Simon Horman <simon.horman@netronome.com>
Signed-off-by: David S. Miller <davem@davemloft.net>