net: lantiq: use netif_tx_napi_add() for TX NAPI

netif_tx_napi_add() should be used for NAPI in the TX direction instead
of the netif_napi_add() function.

Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>
Signed-off-by: David S. Miller <davem@davemloft.net>