net: bcmgenet: rework Tx NAPI code

Introduce new bcmgenet functions to handle the NAPI calls to:
netif_napi_add()
napi_enable()
napi_disable()
netif_napi_del()

Signed-off-by: Petri Gynther <pgynther@google.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>