net: bcmgenet: improve bcmgenet_mii_setup()

bcmgenet_mii_setup() is called from the PHY state machine every 1-2 seconds
when the PHYs are in PHY_POLL mode.

Improve bcmgenet_mii_setup() so that it touches the MAC registers only when
the link is up and there was a change to link, speed, duplex, or pause status.

Signed-off-by: Petri Gynther <pgynther@google.com>
Tested-by: Florian Fainelli <f.fainelli@gmail.com>
Acked-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>