net: phy: mscc: factorize part of config function for VSC8584

Part of the config is common between the VSC8584 and the VSC8574, so to
prepare for the upcoming support of VSC8574, use the phy_device.priv
pointer that will keep the function that holds code that is PHY-specific
and that should be called during config function.

Signed-off-by: Quentin Schulz <quentin.schulz@bootlin.com>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>