net: dsa: qca8k: drop MTU tracking from qca8k_priv

DSA set the CPU port based on the largest MTU of all the slave ports.
Based on this we can drop the MTU array from qca8k_priv and set the
port_change_mtu logic on DSA changing MTU of the CPU port as the switch
have a global MTU settingfor each port.

Signed-off-by: Ansuel Smith <ansuelsmth@gmail.com>
Reviewed-by: Vladimir Oltean <olteanv@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>