net: dsa: rtl8366rb: Support setting MTU

This implements the missing MTU setting for the RTL8366RB
switch.

Apart from supporting jumboframes, this rids us of annoying
boot messages like this:
realtek-smi switch: nonfatal error -95 setting MTU on port 0

Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>