net: dsa: mv88e6xxx: Allow dynamic reconfiguration of tag protocol

For devices that supports both regular and Ethertyped DSA tags, allow
the user to change the protocol.

Additionally, because there are ethernet controllers that do not
handle regular DSA tags in all cases, also allow the protocol to be
changed on devices with undocumented support for EDSA. But, in those
cases, make sure to log the fact that an undocumented feature has been
enabled.

Signed-off-by: Tobias Waldekranz <tobias@waldekranz.com>
Reviewed-by: Vladimir Oltean <olteanv@gmail.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>