bcm63xx_enet: fix poll callback.

In case there was some tx buffer reclaimed and not enough rx packets
to consume the whole budget, napi_complete would not be called and
interrupts would be kept disabled, effectively resulting in the
network core never to call the poll callback again and no rx/tx
interrupts to be fired either.

Fix that by only accounting the rx work done in the poll callback.

Signed-off-by: Nicolas Schichan <nschichan@freebox.fr>
Acked-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>