net: dsa: vsc73xx: fix MTU configuration

Switch in MAXLEN register stores the maximum size of a data frame.
The MTU size is 18 bytes smaller than the frame size.

The current settings are causing problems with packet forwarding.
This patch fixes the MTU settings to proper values.

Fixes: fb77ffc6

 ("net: dsa: vsc73xx: make the MTU configurable")
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Pawel Dembicki <paweldembicki@gmail.com>
Reviewed-by: Vladimir Oltean <olteanv@gmail.com>
Link: https://lore.kernel.org/r/20230628194327.1765644-1-paweldembicki@gmail.com


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