net: dsa: realtek: fix digital interface select macro for EXT0

While no supported devices currently utilize EXT0, the register reserves
the bits for an EXT0. EXT0 is utilized by devices from the generation
prior to rtl8365mb, such as those supported by the driver library
rtl8367b.

Signed-off-by: Luiz Angelo Daros de Luca <luizluca@gmail.com>
Reviewed-by: Alvin Šipraga <alsi@bang-olufsen.dk>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Link: https://lore.kernel.org/r/20240212-realtek-fix_ext0-v1-1-f3d2536d191a@gmail.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>