net/mlx4: fix invalid errno value sign

net/mlx4: fix invalid errno value sign

Tx queue elements allocation function sets rte_errno properly and returns
its negative version. Reassigning this value to rte_errno is thus both
invalid and unnecessary.

Fixes: 9d14b27308a0 ("net/mlx4: standardize on negative errno values")

Signed-off-by: Adrien Mazarguil <adrien.mazarguil@6wind.com>
Acked-by: Nelio Laranjeiro <nelio.laranjeiro@6wind.com>