net: bcmgenet: rework bcmgenet_netif_start and bcmgenet_netif_stop

This commit consolidates more common functionality from
bcmgenet_close and bcmgenet_suspend into bcmgenet_netif_stop and
modifies the start and stop sequences to better suit the design
of the GENET hardware.

Signed-off-by: Doug Berger <opendmb@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>