dpdk: rte_eth_tx_prepare need to called before transmiting packets

dpdk: rte_eth_tx_prepare need to called before transmiting packets

By the DPDK documentation rte_eth_tx_prepare() call is required
to prepare the NIC and to validate the mbufs.

This would fix hardware bad UDP checksum generated by ena driver issue.

As performance will be impacted depending on the driver tx_prepare() callback
I defined a tx-prepare flag that would be enabled by default on ena driver but
could be enabled in the configuration if need for others drivers.

Note: This option would normally be exclusive with intel_phdr_cksum as driver's
tx_prepare would normally cover this usage.

Type: fix
Change-Id: Ic7c21682f7bd92b35bd9b1028129709baa2a64d4
Signed-off-by: Nicolas PLANEL <[email protected]>
Signed-off-by: Nicolas PLANEL <[email protected]>