net/failsafe: change back-reference from sub-device

In multiprocess context, the sub-device structure is shared
between processes. The reference to the failsafe device was
a per process pointer. It's changed to port id which is the
same for all processes.

Signed-off-by: Raslan Darawsheh <rasland@mellanox.com>
Signed-off-by: Thomas Monjalon <thomas@monjalon.net>
Acked-by: Gaetan Rivet <gaetan.rivet@6wind.com>