bcm63xx_enet: Stop pretending to support netpoll

bcm_enet_netpoll does not exist, and causing
bcm63xx_net to fail to build when NET_POLL_CONTROLLER
is defined.

Remove the bogus .ndo_poll_controller = bcm_enet_netpoll

Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>
Signed-off-by: David S. Miller <davem@davemloft.net>