net: dsa: qca8k: Fix port MTU setting

The qca8k only supports a switch-wide MTU setting, and the code to take
the max of all ports was only looking at the port currently being set.
Fix to examine all ports.

Reported-by: DENG Qingfang <dqfext@gmail.com>
Fixes: f58d2598

 ("net: dsa: qca8k: implement the port MTU callbacks")
Signed-off-by: Jonathan McDowell <noodles@earth.li>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Link: https://lore.kernel.org/r/20201030183315.GA6736@earth.li


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