distributor: fix crash in flush

distributor: fix crash in flush

Occasionally, the distributor single-packet-at-a-time mode will
segfault because it inadvertently calls some burst mode code when
flushing packets.

This patch ensures that only the v20 (single mode) codepath is used, and
returns without falling through to the burst mode code.

Fixes: 775003ad2f96 ("distributor: add new burst-capable library")

Signed-off-by: David Hunt <david.hunt@intel.com>
Tested-by: Yong Liu <yong.liu@intel.com>
Acked-by: Bruce Richardson <bruce.richardson@intel.com>