net: dsa: Allow the DSA driver to indicate the tag protocol

DSA drivers may drive different families of switches which need
different tag protocol. Rather than hard code the tag protocol in the
driver structure, have a callback for the DSA core to call.

Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Reviewed-by: Vivien Didelot <vivien.didelot@savoirfairelinux.com>
Signed-off-by: David S. Miller <davem@davemloft.net>