net: dsa: Expose tagging protocol to user-space

There is no way for user-space to know what a given DSA network device's
tagging protocol is. Expose this information through a dsa/tagging
attribute which reflects the tagging protocol currently in use.

This is helpful for configuration (e.g: none behaves dramatically
different wrt. bridges) as well as for packet capture tools when there
is not a proper Ethernet type available.

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