net/mlx5: fix invalid network interface index

net/mlx5: fix invalid network interface index

Network interface indices being unsigned, an invalid index or error is
normally expressed through a zero value (see if_nametoindex()).

mlx5_ifindex() has a signed return type for negative values in case of
error. Since mlx5_nl.c does not check for errors, these may be fed back as
invalid interfaces indices to subsequent system calls. This usage would
have been correct if mlx5_ifindex() returned a zero value instead.

This patch makes mlx5_ifindex() unsigned for convenience.

Fixes: ccdcba53a3f4 ("net/mlx5: use Netlink to add/remove MAC addresses")
Cc: stable@dpdk.org
Signed-off-by: Adrien Mazarguil <adrien.mazarguil@6wind.com>
Acked-by: Nelio Laranjeiro <nelio.laranjeiro@6wind.com>
Acked-by: Yongseok Koh <yskoh@mellanox.com>