s2io.c: Convert skipped nic->config.tx_cfg[i]. to tx_cfg->

Missed doing the conversion in earlier patch.

Signed-off-by: Joe Perches <joe@perches.com>
Acked-by: Sreenivasa Honnur <sreenivasa.honnur@neterion.com>
Signed-off-by: David S. Miller <davem@davemloft.net>