net: dsa: qca8k: implement the port MTU callbacks

This switch has a single max frame size configuration register, so we
track the requested MTU for each port and apply the largest.

v2:
- Address review feedback from Vladimir Oltean

Signed-off-by: Jonathan McDowell <noodles@earth.li>
Acked-by: Vladimir Oltean <olteanv@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>