eventdev: fix Rx adapter event flush

eventdev: fix Rx adapter event flush

The Rx adapter flushes events only if it has BATCH_SIZE
events buffered where BATCH_SIZE is set to 32, e.g., if a
single packet is sent, it is never passed to
eventdev. Fix this issue by adding an event buffer flush
either when a Rx queue is found to be empty or the adapter service
function has processed the max number of packets for an invocation.

Bugzilla ID: 277
Fixes: 6b83f5935543 ("eventdev: add event buffer flush in Rx adapter")
Cc: stable@dpdk.org
Reported-by: Matias Elo <matias.elo@nokia.com>
Signed-off-by: Nikhil Rao <nikhil.rao@intel.com>
Reviewed-by: Mattias Rönnblom <mattias.ronnblom@ericsson.com>
Tested-by: Matias Elo <matias.elo@nokia.com>