mlx5: avoid unused variable warning

When CONFIG_NET_CLS_ACT is disabled, we get a new warning in the mlx5
ethernet driver because the tc_for_each_action() loop never references
the iterator:

mellanox/mlx5/core/en_tc.c: In function 'mlx5e_stats_flower':
mellanox/mlx5/core/en_tc.c:431:20: error: unused variable 'a' [-Werror=unused-variable]
  struct tc_action *a;

This changes the dummy tc_for_each_action() macro by adding a
cast to void, letting the compiler know that the variable is
intentionally declared but not used here. I could not come up
with a nicer workaround, but this seems to do the trick.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Fixes: aad7e08d ("net/mlx5e: Hardware offloaded flower filter statistics support")
Fixes: 00175aec ("net/sched: Macro instead of CONFIG_NET_CLS_ACT ifdef")
Acked-By: Amir Vadai <amir@vadai.me>
Signed-off-by: David S. Miller <davem@davemloft.net>