nfp: flower: check that we don't exceed the FW key size

Add a check to make sure the total length of the flow key sent to the
firmware stays within the supported limit.

Signed-off-by: Louis Peens <louis.peens@netronome.com>
Signed-off-by: Simon Horman <simon.horman@netronome.com>
Signed-off-by: David S. Miller <davem@davemloft.net>