net/failsafe: fix Rx interrupt reinstallation

net/failsafe: fix Rx interrupt reinstallation

Fail-safe dev_start() operation can be called by both the application
and the hot-plug alarm mechanism.

The installation of Rx interrupt are triggered from dev_start() in any
time it is called while actually the Rx interrupt should be installed
only by the application calls.

So, each plug-in event causes reinstallation which causes memory leak
and spoils the fail-safe Rx interrupt mechanism.

Trigger the Rx interrupt installation only when it does not exist.

Fixes: 9e0360aebf23 ("net/failsafe: register as Rx interrupt mode")
Signed-off-by: Matan Azrad <matan@mellanox.com>
Acked-by: Gaetan Rivet <gaetan.rivet@6wind.com>