net: dsa: mv88e6xxx: Move the tagging protocol into info

Older chips support a single tagging protocol, DSA. New chips support
both DSA and EDSA, an enhanced version. Having both as an option
changes the register layouts. Up until now, it has been assumed that
if EDSA is supported, it will be used. Hence the register layout has
been determined by which protocol should be used. However, mv88e6390
has a different implementation of EDSA, which requires we need to use
the DSA tagging. Hence separate the selection of the protocol from the
register layout.

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>