mlxsw: switchx2: Don't count internal TX header bytes to stats

Stop the SW TX counter from counting the TX header bytes
since they are not being sent out.

Fixes: e577516b ("mlxsw: Fix use-after-free bug in mlxsw_sx_port_xmit")
Signed-off-by: Nogah Frankel <nogahf@mellanox.com>
Reviewed-by: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>