nfp: flower: fix implicit fallthrough warning

The nfp_flower_copy_pre_actions function introduces a case statement with
an intentional fallthrough. However, this generates a warning if built
with the -Wimplicit-fallthrough flag.

Remove the warning by adding a fall through comment.

Fixes: 1c6952ca

 ("nfp: flower: generate merge flow rule")
Signed-off-by: John Hurley <john.hurley@netronome.com>
Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Reviewed-by: Jiong Wang <jiong.wang@netronome.com>
Reviewed-by: Simon Horman <simon.horman@netronome.com>
Signed-off-by: David S. Miller <davem@davemloft.net>