bgmac: start/stop PHY on netdev open/stop

I've realized that I need to call ethtool command to get Ethernet
working after booting. Ex call: ethtool -s eth0 autoneg on
It was fixing Ethernet even if auto-negotiation was already on.

Adding calls to phy_start and phy_stop look like a real solution.

Signed-off-by: Rafał Miłecki <zajec5@gmail.com>
Acked-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>