net/failsafe: fix Rx burst infinite loop

In case of plugged out device, the fail-safe PMD uses failsafe_rx_burst
function for packet receiving.

This function iterates over the present sub-devices until it
receives a traffic from one of them or they are all cannot receive
packets.

The corrupted code didn't advance the sub-device pointer when the
sub-device was not present and caused to infinite loop.

Advance the sub-device pointer also in plugged-out sub-device case.

Fixes: 8052bbd9d548 ("net/failsafe: improve Rx sub-devices iteration")

Signed-off-by: Matan Azrad <matan@mellanox.com>
Acked-by: Gaetan Rivet <gaetan.rivet@6wind.com>