net: systemport: add bcm_sysport_netif_{enable,stop}

Factor common code that either enables or disables the network
interface with the networking stack. We are going to reuse these
functions for suspend/resume callbacks.

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