net: dsa: Fix tagging attribute location

While introducing the DSA tagging protocol attribute, it was added to the DSA
slave network devices, but those actually see untagged traffic (that is their
whole purpose). Correct this mistake by putting the tagging sysfs attribute
under the DSA master network device where this is the information that we need.

While at it, also correct the sysfs documentation mistake that missed the
"dsa/" directory component of the attribute.

Fixes: 98cdb480 ("net: dsa: Expose tagging protocol to user-space")
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>