net: dsa: bcm_sf2: disable RGMII interface(s) when link is down

When the link is down, disable the RGMII interface to conserve as much
power as possible. We re-enable the RGMII interface whenever the link is
detected.

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