net: bcmgenet: Remove custom ndo_poll_controller()

The driver gained a .ndo_poll_controller() at a time where the TX
cleaning process was always done from NAPI which makes this unnecessary.
See commit ac3d9dd0

 ("netpoll: make ndo_poll_controller() optional")
for more background.

Signed-off-by: Florian Fainelli <florian.fainelli@broadcom.com>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>