net/mlx5: fix build with gcc 7.1

net/mlx5: fix build with gcc 7.1

build error:
.../dpdk/drivers/net/mlx5/mlx5_fdir.c:
  In function ‘fdir_filter_to_flow_desc’:
.../dpdk/drivers/net/mlx5/mlx5_fdir.c:146:18:
 error: this statement may fall through [-Werror=implicit-fallthrough=]
   desc->dst_port = fdir_filter->input.flow.udp4_flow.dst_port;
   ~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
.../dpdk/drivers/net/mlx5/mlx5_fdir.c:147:2: note: here
  case RTE_ETH_FLOW_NONFRAG_IPV4_OTHER:
  ^~~~

Fixed by adding fallthrough comment to the code.

Fixes: 76f5c99e6840 ("mlx5: support flow director")
Cc: stable@dpdk.org
Signed-off-by: Ferruh Yigit <ferruh.yigit@intel.com>
Acked-by: Adrien Mazarguil <adrien.mazarguil@6wind.com>