net: dsa: qca8k: reset cpu port on MTU change

net: dsa: qca8k: reset cpu port on MTU change

It was discovered that the Documentation lacks of a fundamental detail
on how to correctly change the MAX_FRAME_SIZE of the switch.

In fact if the MAX_FRAME_SIZE is changed while the cpu port is on, the
switch panics and cease to send any packet. This cause the mgmt ethernet
system to not receive any packet (the slow fallback still works) and
makes the device not reachable. To recover from this a switch reset is
required.

To correctly handle this, turn off the cpu ports before changing the
MAX_FRAME_SIZE and turn on again after the value is applied.

Fixes: 596135cb52e3 ("net: dsa: qca8k: implement the port MTU callbacks")
Cc: stable@vger.kernel.org
Signed-off-by: Christian Marangi <ansuelsmth@gmail.com>
Link: https://lore.kernel.org/r/20220621151122.10220-1-ansuelsmth@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>