net: phy: export genphy_config_init()

This enables other drivers to call this generic implementation, and then
only do specific details on top of it.

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