net: phy: realtek: register/unregister multiple drivers properly

Using phy_drivers_register/_unregister functions is proper way to
handle multiple PHY drivers registration. For Realtek PHY drivers
module, it fixes incomplete current error-handlings up and adds
missed unregistration for the RTL8201CP driver.

Signed-off-by: Jongsung Kim <neidhard.kim@lge.com>
Signed-off-by: David S. Miller <davem@davemloft.net>