event/octeontx2: fix unconditional Tx flush

Fix unconditional Tx flush, in case of Tx only we need to check if
work slot is non-empty before issuing flush.
Also, in packet retransmit cases added check for the reference
count and flush the work slot only for the last packet.

Fixes: cb7ee83b6365 ("event/octeontx2: improve single flow performance")
Cc: stable@dpdk.org
Signed-off-by: Pavan Nikhilesh <pbhagavatula@marvell.com>