ethdev: fix Tx prepare documentation to use positive errno

[ upstream commit aab0afb ]

Tx prepare documentation was incorrectly suggesting to use
negative rte_errno.

Fixes: 4fb7e80 ("ethdev: add Tx preparation")
Fixes: 439a90b ("ethdev: reorder inline functions")

Signed-off-by: Andrew Rybchenko <arybchenko@solarflare.com>
Reviewed-by: Ferruh Yigit <ferruh.yigit@intel.com>

lib/librte_ethdev/rte_ethdev.h

@@ -4206,8 +4206,8 @@ rte_eth_tx_burst(uint16_t port_id, uint16_t queue_id,
  *   The number of packets correct and ready to be sent. The return value can be
  *   less than the value of the *tx_pkts* parameter when some packet doesn't
  *   meet devices requirements with rte_errno set appropriately:
- *   - -EINVAL: offload flags are not correctly set
- *   - -ENOTSUP: the offload feature is not supported by the hardware
+ *   - EINVAL: offload flags are not correctly set
+ *   - ENOTSUP: the offload feature is not supported by the hardware
  *
  */