mlx5: unitialized warning

[Stephen Hemminger]

Building current main branch with gcc (Debian 10.2.1-6) 10.2.1 20210110


[1843/3131] Compiling C object drivers/libtmp_rte_net_mlx5.a.p/net_mlx5_mlx5_flow_meter.c.o
../drivers/net/mlx5/mlx5_flow_meter.c: In function ‘mlx5_flow_meter_create’:
../drivers/net/mlx5/mlx5_flow_meter.c:1170:33: warning: ‘legacy_fm’ may be used uninitialized in this function [-Wmaybe-uninitialized]
 1170 |  struct mlx5_legacy_flow_meter *legacy_fm;
      |                                 ^~~~~~~~~