net: dsa: realtek: rtl8366rb: Serialize indirect PHY register access

Lock the regmap during the whole PHY register access routines in
rtl8366rb.

Signed-off-by: Alvin Šipraga <alsi@bang-olufsen.dk>
Tested-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Reviewed-by: Vladimir Oltean <olteanv@gmail.com>
Link: https://lore.kernel.org/r/20220513213618.2742895-1-linus.walleij@linaro.org


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