net: bcmgenet: Allow changing carrier from user-space

The GENET driver interfaces with internal MoCA interface as well as
external MoCA chips like the BCM6802/6803 through a fixed link
interface. It is desirable for the mocad user-space daemon to be able to
control the carrier state based upon out of band messages that it
receives from the MoCA chip.

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