net: dsa: remove dp->priv

All current in-tree uses of dp->priv have been replaced with
ds->tagger_data, which provides for a safer API especially when the
connection isn't the regular 1:1 link between one switch driver and one
tagging protocol driver, but could be either one switch to many taggers,
or many switches to one tagger.

Therefore, we can remove this unused pointer.

Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>