net/enic: set rte errno to positive value

net/enic: set rte errno to positive value

Related to d9fff8a31, where rte_errno should always have positive
errno values.

Technically this is an ABI change since it fixes an error code
introduced in 18.02, but is minor and inconsequential.

Fixes: 1e81dbb5321b ("net/enic: add Tx prepare handler")
Cc: stable@dpdk.org
Signed-off-by: Hyong Youb Kim <hyonkim@cisco.com>
Reviewed-by: John Daley <johndale@cisco.com>
Reviewed-by: Aaron Conole <aconole@redhat.com>