event/dsw: flush buffers immediately on zero-sized enqueue

event/dsw: flush buffers immediately on zero-sized enqueue

Fix DSW's rte_event_enqueue_burst(), so that a call with a zero-sized
event array immediately flushes the port's output buffers. Prior to
this patch, the flush operation would be deferred to the next enqueue
or dequeue call, which is inconsistent with DSW documentation.

Fixes: 1c8e3caa3bfb ("event/dsw: add event scheduling and device start/stop")
Cc: stable@dpdk.org
Signed-off-by: Mattias Rönnblom <mattias.ronnblom@ericsson.com>