net: dsa: tag_sja1105: fix zeroization of ds->priv on tag proto disconnect

The method was meant to zeroize ds->tagger_data but got the wrong
pointer. Fix this.

Fixes: c79e8486 ("net: dsa: tag_sja1105: convert to tagger-owned data")
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>