MAINTAINERS: rectify entry for REALTEK RTL83xx SMI DSA ROUTER CHIPS

Commit 429c83c7 ("dt-bindings: net: dsa: realtek: convert to YAML
schema, add MDIO") converts realtek-smi.txt to realtek.yaml, but missed to
adjust its reference in MAINTAINERS.

Hence, ./scripts/get_maintainer.pl --self-test=patterns complains about a
broken reference.

Repair this file reference in REALTEK RTL83xx SMI DSA ROUTER CHIPS.

Signed-off-by: Lukas Bulwahn <lukas.bulwahn@gmail.com>
Reviewed-by: Vladimir Oltean <olteanv@gmail.com>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Reviewed-by: Alvin Šipraga <alsi@bang-olufsen.dk>
Signed-off-by: David S. Miller <davem@davemloft.net>