net/core: tidy up an error message

netif_napi_add() could report an error like this below due to it allows
to pass a format string for wildcarding before calling
dev_get_valid_name(),

"netif_napi_add() called with weight 256 on device eth%d"

For example, hns_enet_drv module does this.

hns_nic_try_get_ae
  hns_nic_init_ring_data
    netif_napi_add
  register_netdev
    dev_get_valid_name

Hence, make it a bit more human-readable by using netdev_err_once()
instead.

Signed-off-by: Qian Cai <cai@gmx.us>
Signed-off-by: David S. Miller <davem@davemloft.net>