net: bcmgenet: modularize bcmgenet_{open,close}

Introduce a bunch of helper functions: bcmgenet_netif_start,
bcmgenet_netif_stop and bcmgenet_intr_disable to help reuse code that is
going to be necessary for suspend/resume.

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