net: dsa: program VLAN on CPU port from slave

DSA currently programs a VLAN on the CPU port implicitly after the
related notifier is received by a switch.

While we still need to do this transparent programmation of the DSA
links in the fabric, programming the CPU port this way may cause
problems in some corners such as the tag_8021q driver.

Because the dedicated CPU port is specific to a slave, make their
programmation explicit a few layers up, in the slave code.

Note that technically, DSA links have a dedicated CPU port as well,
but since they are only used as conduit between interconnected switches
of a fabric, programming them transparently this way is what we want.

Signed-off-by: Vivien Didelot <vivien.didelot@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>