net: stmmac: fix out-of-bounds access in a selftest

net: stmmac: fix out-of-bounds access in a selftest

GCC 12 points out that struct tc_action is smaller than
struct tcf_action:

drivers/net/ethernet/stmicro/stmmac/stmmac_selftests.c: In function ‘stmmac_test_rxp’:
drivers/net/ethernet/stmicro/stmmac/stmmac_selftests.c:1132:21: warning: array subscript ‘struct tcf_gact[0]’ is partly outside array bounds of ‘unsigned char[272]’ [-Warray-bounds]
 1132 |                 gact->tcf_action = TC_ACT_SHOT;
      |                     ^~

Fixes: ccfc639a94f2 ("net: stmmac: selftests: Add a selftest for Flexible RX Parser")
Link: https://lore.kernel.org/r/20220519004305.2109708-1-kuba@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>