rte_errno should be set to positive value from errno.h plus
few RTE-specific values.
Fixes: 4fb7e803eb1a ("ethdev: add Tx preparation")
Fixes: 439a90b5f2a7 ("ethdev: reorder inline functions")
Cc: stable@dpdk.org
Signed-off-by: Andrew Rybchenko <arybchenko@solarflare.com>