net: dsa: vsc73xx: fix MTU configuration

net: dsa: vsc73xx: fix MTU configuration

[ Upstream commit 3cf62c8177adb0db9e15c8b898c44f997acf3ebf ]

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: fb77ffc6ec86 ("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>
Signed-off-by: Sasha Levin <sashal@kernel.org>