net_sched: fix unused variable warning in stmmac

The new tcf_exts_for_each_action() macro doesn't reference its
arguments when CONFIG_NET_CLS_ACT is disabled, which leads to
a harmless warning in at least one driver:

drivers/net/ethernet/stmicro/stmmac/stmmac_tc.c: In function 'tc_fill_actions':
drivers/net/ethernet/stmicro/stmmac/stmmac_tc.c:64:6: error: unused variable 'i' [-Werror=unused-variable]

Adding a cast to void lets us avoid this kind of warning.
To be on the safe side, do it for all three arguments, not
just the one that caused the warning.

Fixes: 244cd96a ("net_sched: remove list_head from tc_action")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: David S. Miller <davem@davemloft.net>