The MTU callbacks are in layer 1 size, so for example 1500
bytes is a normal setting. Cache this size, and only add
the layer 2 framing right before choosing the setting. On
the CPU port this will however include the DSA tag since
this is transmitted from the parent ethernet interface!
Add the layer 2 overhead such as ethernet and VLAN framing
and FCS before selecting the size in the register.
This will make the code easier to understand.
The rtl8366rb_max_mtu() callback returns a bogus MTU
just subtracting the CPU tag, which is the only thing
we should NOT subtract. Return the correct layer 1
max MTU after removing headers and checksum.