net/mlx4: fix assertion failure on link update

The interrupt handler can sometimes be triggered for reasons other than a
link status event. An assertion failure happen when such events occur while
an asynchronous link status update is already scheduled.

Address this issue using the same approach as its mlx5 counterpart,
commit a9f2fbc42f0c ("net/mlx5: fix inconsistent link status")

Fixes: c4da6caa426d ("mlx4: handle link status interrupts")
Cc: stable@dpdk.org
Signed-off-by: Adrien Mazarguil <adrien.mazarguil@6wind.com>
Acked-by: Nelio Laranjeiro <nelio.laranjeiro@6wind.com>