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>