net: dsa: let switches specify their tagging protocol

In order to support the new DSA device driver model, a dsa_switch should
be able to advertise the type of tagging protocol supported by the
underlying switch device. This also removes constraints on how tagging
can be stacked to each other.

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>