net/ena: enable multi-segment in Tx offload flags

net/ena: enable multi-segment in Tx offload flags

The DPDK ENA driver does not provide multi-segment tx offload capability.
Let's add DEV_TX_OFFLOAD_MULTI_SEGS to ports offload capability by
default, and always set it in dev->data->dev_conf.txmode.offload.

This flag in not listed in doc/guides/nics/features/default.ini, so
ena.ini does not need to be updated.

Fixes: 1173fca25af9 ("ena: add polling-mode driver")
Cc: stable@dpdk.org
Signed-off-by: Ghalem Boudour <ghalem.boudour@6wind.com>
Signed-off-by: Olivier Matz <olivier.matz@6wind.com>
Acked-by: Michal Krawczyk <mk@semihalf.com>