net: dsa: realtek: keep variant reference in realtek_priv

Instead of copying values from the variant, we can keep a reference in
realtek_priv.

This is a preliminary change for sharing code betwen interfaces. It will
allow to move most of the probe into a common module while still allow
code specific to each interface to read variant fields.

Signed-off-by: Luiz Angelo Daros de Luca <luizluca@gmail.com>
Reviewed-by: Vladimir Oltean <olteanv@gmail.com>
Reviewed-by: Florian Fainelli <florian.fainelli@broadcom.com>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: David S. Miller <davem@davemloft.net>