ethdev: add function to look up Tx offload names

ethdev: add function to look up Tx offload names

Commonly, drivers converted to the new offload API
may need to log unsupported offloads as a response
to wrong settings. From this perspective, it would
be convenient to have generic functions to look up
offload names. The patch adds such a helper for Tx.

Signed-off-by: Ivan Malov <ivan.malov@oktetlabs.ru>
Signed-off-by: Andrew Rybchenko <arybchenko@solarflare.com>
Reviewed-by: Ferruh Yigit <ferruh.yigit@intel.com>
Acked-by: Thomas Monjalon <thomas@monjalon.net>