net: stmmac: Use netif_tx_napi_add() for TX polling function

This variant of netif_napi_add() should be used from drivers
using NAPI to exclusively poll a TX queue.

Signed-off-by: Frode Isaksen <fisaksen@baylibre.com>
Tested-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>
Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>
Signed-off-by: David S. Miller <davem@davemloft.net>