ixgbe: rename Tx queue release function

ixgbe: rename Tx queue release function

The function inside the vector/SSE poll-mode driver for releasing
the mbufs on the TX queues had the same name as another function
inside the regular PMD. To keep consistency and avoid confusion,
rename the vector PMD version to have a "_vec" suffix.

Signed-off-by: Bruce Richardson <bruce.richardson@intel.com>
Signed-off-by: Konstantin Ananyev <konstantin.ananyev@intel.com>
Acked-by: Helin Zhang <helin.zhang@intel.com>