net: dsa: tag_qca: fix doubled Tx statistics

DSA subsystem takes care of netdev statistics since commit 4ed70ce9
("net: dsa: Refactor transmit path to eliminate duplication"), so
any accounting inside tagger callbacks is redundant and can lead to
messing up the stats.
This bug is present in Qualcomm tagger since day 0.

Fixes: cafdc45c

 ("net-next: dsa: add Qualcomm tag RX/TX handler")
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Alexander Lobakin <alobakin@dlink.ru>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>