net: bcmgenet: pull mac_config from adjust_link

This commit separates out the MAC configuration that occurs on a
PHY state change into a function named bcmgenet_mac_config().

This allows the function to be called directly elsewhere.

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