net: if: Create support to flush the TX queue

If we are taking network interface down, then flush the TX for any
pending bufs that would be needed to be sent.

Change-Id: I229ecce8e07ec5847d4c6c6a1994de04bd9171e2
Signed-off-by: Jukka Rissanen <jukka.rissanen@linux.intel.com>