docs: net: dsa: rename tag_protocol to get_tag_protocol

Since the blamed commit, the enum was turned into a function pointer and
also renamed. Update the documentation.

Fixes: 7b314362 ("net: dsa: Allow the DSA driver to indicate the tag protocol")
Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

Documentation/networking/dsa/dsa.rst

@@ -593,8 +593,8 @@ driver may free the data structures associated with the ``dsa_switch``.
 Switch configuration
 --------------------
 
-- ``tag_protocol``: this is to indicate what kind of tagging protocol is supported,
-  should be a valid value from the ``dsa_tag_protocol`` enum
+- ``get_tag_protocol``: this is to indicate what kind of tagging protocol is
+  supported, should be a valid value from the ``dsa_tag_protocol`` enum
 
 - ``setup``: setup function for the switch, this function is responsible for setting
   up the ``dsa_switch_ops`` private structure with all it needs: register maps,