net/mlx5: allow unknown link speed

mlx5 PMD refuses to update link state if link speed is defined but
status is down or if link speed is undefined but status is up, even if
the ioctl() succeeded.
This prevents application to detect link up/down event, especially when
the link speed is not correctly detected.

Commit [1] allowed returning unknown link speed, so now PMD allows
the return of unknown link speed in the above case.

Due to some old kernel driver bug, link speed wasn't detected properly.

[1] http://git.dpdk.org/dpdk/commit/?id=810b17d116f03

Signed-off-by: Benoît Ganne <bganne@cisco.com>
Signed-off-by: Raslan Darawsheh <rasland@nvidia.com>
Acked-by: Matan Azrad <matan@nvidia.com>