net/mlx4: standardize on negative errno values

net/mlx4: standardize on negative errno values

Due to its reliance on system calls, the mlx4 PMD uses positive errno
values internally and negative ones at the ethdev API border. Although most
internal functions are documented, this mixed design is unusual and prone
to mistakes (e.g. flow API implementation uses negative values
exclusively).

Standardize on negative errno values and rely on rte_errno instead of
errno in all functions.

Signed-off-by: Adrien Mazarguil <adrien.mazarguil@6wind.com>