net: enetc: reset taprio stats when taprio is deleted

Currently, the window_drop stats persist even if an incorrect Qdisc was
removed from the interface and a new one is installed. This is because
the enetc driver keeps the state, and that is persistent across multiple
Qdiscs.

To resolve the issue, clear all win_drop counters from all TX queues
when the currently active Qdisc is removed. These counters are zero
by default. The counters visible in ethtool -S are also affected,
but I don't care very much about preserving those enough to keep them
monotonically incrementing.

Fixes: 4802fca8 ("net: enetc: report statistics counters for taprio")
Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>