net: ethernet: ti: cpts: purge staled skbs from txq

The overflow event is running with 1 jiffy in case if txq is not
empty, but it can be emptied completely only if next tx event
consumes skb or deletes staled skb from the txq. In case of staled
skb, that can happen for some unpredictable reason (the ts event was
lost or timed out), the overflow event can be generated quite long
time consuming CPU w/o reason before next tx event happens. To avoid
it, purge txq before increasing overflow event rate.

Signed-off-by: Ivan Khoronzhuk <ivan.khoronzhuk@linaro.org>
Signed-off-by: David S. Miller <davem@davemloft.net>