dfc: reset tx queue

When a bearer is removed, calling qdisc reset on a tx queue
could have a race condition with qdisc dequeue for lockless
qdisc such as pfifo_fast.

This change uses a different mechanism not relying on qdisc
implementation to achieve packet purge on bearer remove.

Change-Id: I8f9201809853b07293896d6cb8e010e9e0904e46
Signed-off-by: Weiyi Chen <quic_weiyic@quicinc.com>